Do JW believe in the Trinity? If yes, why and if no, why
No they do not. The reason goes back to the Bible Student founder Charles Taze Russell and his core tenet of the ransom sacrifice: A perfect human Jesus in exchange for a perfect Adam.
This allows for humans to be resurrected from the dead to a paradise earth because the scales of universal justice have supposedly been balanced.
However Russell still left a problem unresolved: If Jesus was just a human how could his redemptive power be used to raise some humans from the dead to heaven? A human ransom alone would not cover it.
The only solution would be if Jesus was both human and God at the same time.
However this is all just fantasy. Its like debating who would win in a fight: Spiderman or Silver Surfer