I replaced the green board around my bath tub. Do I have to paint it before I install the tub surround

No you don't have to paint the green board. You should seal the joints with at least a tape coat of spackle.

I,am installing a 5 piece tub surround and the end will is not straight so my corner piece will not fit to the wall i.ts OK at the bottom but is out at the top by 3in how do I cut this so it will fit