In twelve angry men why is it so important for three that the boy be convicted

Not sure what your question is but I am familiar with Twelve angry men:

Juror #3 was played by actor Lee J Cobb. he was one of the actors in the Exorcist.

The character of Juror #3 had lost his relationship with his son and that is why he wanted to find him guilty.

Juror #3 occupation was the Owner of Messanger Services

(I hope this answers your question - Next time be careful to ask a question so others can answer it in a timely manner)