Why did Tituba, Abigail,
and Betty confess?

If they confessed they would not be hanged if they didn’t they would be hanged.

Tituba confesses because she realizes that it's a lot easier to put the blame on other people, and the only way she can avoid being hanged is to confess to witchcraft. ... Tituba says that the devil wanted her to kill Parris multiple times, but that she kept on refusing, even when the devil promised her return to Barbados.

Abigail She admits that Tituba and Ruth were conjuring spirits only to save Betty from being accused of witchcraft. Abigail treats the other girls badly, but treats her uncle good and with respect.

No adult present has seen Betty behave at all coherently until she rises from her bed, having heard Abigail's accusation of Tituba as well as Tituba's own confession to witchcraft. ... Betty starts accusing people to deflect suspicion away from herself and the other girls from their dabblings in the occult in the woods.