Why did Mary Warren turn against Proctor in Act 3 of The Crucible, despite her previous admissions?

On one hand, Mary Warren turns against Proctor because of her own capitulation to Abigail and the girls.  It becomes evident that Mary needs the affirmation of the group at greater cost than what the truth might be.  The fundamental issue that Mary Warren wrestles with is that she is coming clean in terms of being honest about the girls' doings in the woods and the accusations in the form of court testimony.  Mary Warren is vitally important because she is the only one who can give clear evidence to what the girls were doing.  Proctor knows this and because of it, Mary becomes vital to his motivation of attempting to reject Abigail's claims in court.  When Mary contradicts Abigail and the girls, telling the truth in the process, it becomes evident that she has distanced herself from the group.  It is here where they become merciless in pretending to be afflicted with a curse of Mary's doing and the other girls' mocking her.  In this, Mary, in her emotionally weakened state, makes the call to side with the girls, even thought she knows that in doing so, she is repudiating the truth.  In the end, I think that her motivation of turning against Proctor is out of the fear that she does not want to be alone.  The expulsion from the group was too much for Mary to bear and in this, she turns against Proctor.

Why does Mary Warren turn on John Proctor in The Crucible?

Mary Warren changes her testimony and accuses John Proctor of witchcraft because she is afraid of Abigail Williams, whom Mary has indirectly accused of murder. When it seems clear that the magistrates are taking Mary's honest testimony seriously, Abigail begins to accuse her of witchcraft. Abigail talks to an invisible "yellow bird" as though it were Mary, suggesting that the bird has said that it "want[s] to tear [Abigail's] face." This testimony would have been admitted to the court as spectral evidence: the Puritans believed that a witch could send out her specter (or spirit) in its own shape or even in the shape of an animal to harm her victims. Since only the witch and her intended victim could see the specter, no one else could deny this evidence. As a result of Abigail's masterful acting, Danforth says to Mary,

A little while ago you were afflicted. Now it seems you afflict others; where did you find this power?

Danforth believes Abigail and not Mary. Meanwhile, the girls begin to mimic Mary's speech, John Proctor tells her that "God damns all liars!" and Danforth insists that she has "seen the Devil" and "made a compact with Lucifer." Mary cannot take the pressure, and she very likely fears for her own life. She knows what Abigail is capable of. Suddenly, she turns on John because this seems to be the only way to save herself. Her testimony against him will, of course, be believed (as all testimony that accuses others has been).

What is Mary Warren's motive for accusing John Proctor of witchcraft in "The Crucible"?

Mary is motivated by Abigail. Abigail threatens the girls if they do not follow through with her malicious scheme. Mary is caught up in play acting. She is motivated by all the attention that she and the girls are receiving:

Mary Warren is the Proctors' servant who seems timid and subservient but who finds a powerful role in a kind of people's jury in the courtroom. 

In her refusal to cooperate with John Proctor, Mary seems to be stubborn and rebellious when she insists that she attend the hearings of the court. Mary is bent on attending even at John Proctor's forbidding:

She occasionally dares to defy Proctor, particularly in her insistence that she must attend the hearings

After John Proctor convinces her that she must testify against Abigail in court, Mary does seem to have a conscience and determines to do the right thing. However, Abigail's presence is threatening and Mary soon changes her mind:

She becomes hysterical before the court, however, and soon joins Abigail in pretending that mere is evil witchcraft at work. Her behavior in the court contributes, in part, to John Proctor's arrest.

Mary acts out of fear for her own safety. She does not want to be determined as one who has been pretending. She fears the court and what the court could do to her. She also fears Abigail. She also enjoys the power she has at the hearings. 

In The Crucible, why does Mary Warren turn against John Proctor and what techniques are used?

The hysteria that permeates the town of Salem and the surrounding environs in Arthur Miller's play The Crucible, a story intended as an allegory for the "witch hunts" that took place in the United States during the late 1940s and early 1950s when fears of communism and the Soviet Union gripped much of the nation, causes the region's inhabitants to act irrationally and viciously towards each other. Friends turn on friends; family members turn on family members. Fear of being accused of sorcery is sufficient to cause people to make ill-founded accusations in order to deflect suspicions away from themselves. Such is the atmosphere in which Miller's play takes place. From its opening scene, in which Reverend Parris frantically tries to deal with his daughter's apparent coma following her and her friends' peculiar and highly suspicious activities in the woods, The Crucible depicts a society on the threshold of a Hell of its own making.

As Miller's play progresses, the hysteria reaches a fever pitch, and old grievances become the fodder for new ones, all intended to ensure one's survival. At the center of this drama are Abigail, whose actions precipitate the tragic chain of events, and, increasingly, Mary Warren, Abigail's replacement in the Proctor home following Abigail's dismissal for engaging in sexual relations with John Proctor. Proctor's relationship to Abigail continues to plague him, and he becomes dependent upon Mary's testimony in court to absolve him while indicting Mary's friend, Abigail. Mary knows of Proctor and Abigail's affair, and is caught between her employer and her friend. As Miller depicts the scene late in Act Two:

Mary Warren: . . .Abby'll charge lechery on you, Mr. Proctor!
Proctor: She's [Abigail] told you!
Mary Warren: I have known it, sir. She'll ruin you with it, I know she will.

Proctor is desperate to ensure Mary's testimony will exonerate him in the witch trial that is tearing the town apart. In Act Three, Proctor testifies before the court that Mary is prepared to confess that charges of sorcery, or witchcraft, have been fabricated, and that her previous statements were false. As Mary is questioned by Danforth, however, Abigail and the other girls enter the court room. Danforth questions Abigail regarding Mary's deposition and testimony, and Abigail accuses her now-former friend of lying when Mary denies any kind of Satanic activities. Abigail succeeds in turning the tables on Mary and, by extension, John Proctor. Abigail pretends to be experiencing symptoms of demonic influence, which casts new suspicions upon Mary, who is now, again, suspected of sorcery. Abigail, as we know from the play's previous activities, is far more duplicitous in nature than the more simple-minded Mary Warren, and the latter breaks down, lending credence to Abigail's intrigues.

In short, Mary is sufficiently frightened by Abigail's tactics that her fear overshadows any commitments she has made with Proctor. 

As noted, Miller intended his play as an allegory about the anti-communist paranoia that permeated the nation during the early 1950s. The point he makes in The Crucible, especially during the trials, is that innocent people suffered unjustly because of the paranoia about threats that did not, or only marginally, existed. Honest people felt compelled to accuse others in order to deflect attention away from themselves.

At the end of Act 2 in "The Crucible", John Proctor threatens Mary Warren's life if she does not go to the court to testify to the judges that the girls have been lying. She is extremely hesitatant to go because she fears that Abigail and the girls will charge her with being a witch -- and this is exactly what begins to happen in Act 3. When Mary begins to tell the court the truth, the other girls, lead by Abigail, begin to mimic her every word; therefore, trying to show the judges that Mary is bewitching them. Instead of taking the fall herself, Mary goes against John by telling the court that he is a witch and was forcing her to tell the court that the girls were lying.

What causes tension between John Proctor and Mary Warren in The Crucible?

In Act Two, Mary Warren enters John Proctor's home after spending the day testifying in Salem's court. John Proctor had specifically forbidden her from traveling to Salem and demanded that she remain working at his home. When Mary returns, she explains to John that she is feeling ill after being in court all day. She then gives Elizabeth a poppet that she made while on the witness stand. Mary then says that they have already convicted thirty-nine people and explains her reasoning for accusing citizens of witchcraft. However, Proctor understands that the accusations are false and tells Mary that she will not be allowed to travel to Salem anymore. Mary responds by challenging Proctor's authority and says,

"You must see it, sir, it’s God’s work we do. So I’ll be gone every day for some time. I’m—I am an official of the court..." (Miller 59).

Mary's response indicates that she values her position of authority in the community. The trials give the young women an elevated status throughout Salem, which is something Mary is willing to defend. 

In The Crucible, why does Mary Warren change her mind in court?

In The Crucible, Mary Warren unwillingly comes to court, though she comes prepared to tell the truth--that she and the rest of the girls, led by Abigail, have been lying in court.  She's unwilling because she's afraid of many things: Abigail, the court, undoubtedly her parents, punishment.  She changes her mind, of course, once Abigail turns her own words literally against her and she realizes her immediate future is better with Abigail than with Proctor. She turns back on Proctor, claiming he was the devil and he was forcing her to make a false confession.