It is probably advisable for you to check with your instructor or in your book to see what three justifications you are supposed to know.  This is because there are many different ways to state the justifications for this rule.  Some scholars argue that there are two justifications, others mention more.

For example, courts have justified the rule by saying the rule is needed to uphold the 4th Amendment.  The Supreme Court put this idea forward in Weeks v. US in 1914.  The rationale here is that people have a constitutional right to be free of illegal searches and seizures.  In other words, the exclusionary rule here is protecting our personal right to our privacy.  Its main purpose is to maintain this right.

As another example, courts have justified the rule saying that it is needed in order to deter the police from violating our rights.  The Supreme Court put this idea forward in 1965 in Linkletter v. Walker.  It reasoned that the police would not be forced to obey the law unless the exclusionary rule were in place.

These two are often seen as the only justifications for the exclusionary rule.  It would be possible to say that another justification, though, is to preserve the integrity of the courts.  If the courts were to allow illegally obtained evidence, they would be condoning the illegal actions of the police.  This idea was also put forward in Weeks.

These are three justifications, but there is no guarantee that these are the “right” ones as different textbooks might list different justifications.