Act III, in The Crucible is a turning point, several important points of the plot turn leading to the resolution in Act IV. For example, Proctor confesses to adultery in Act III in an effort to shake Abigail's hold over the court, instead of helping him, it seals his fate.
In order to prove the charge of adultery, Elizabeth Proctor is brought in to verify the claim, she lies to protect her husband's reputation.
Mary Warren turns on Proctor under pressure from Abigail's charade about seeing a bird flying in the court, that she claims is Mary's spirit. Mary accuses Proctor of trying to force her to follow the devil.
Proctor is arrested and thrown in jail along with Giles Corey, who refuses to name the individual who told him about Thomas Putnam instructing his daughter Ruth to accuse George Jacobs of witchcraft.
Reverend Hale quits the court and leaves Salem at the end of Act III, this is a very significant turning point. Hale becomes convinced that the court is being used as an instrument of vengeance by members of the town and that the truth has been subverted in favor of maintaining the court's authority.
The actions of the characters in Act III, lead up to the resolution that occurs in Act IV.