I have some projectiles that I want to fire all at once. I want one to fire straight and the others to fire at angles from the player. I'm assuming I should use vectors to calculate these angles but I'm not sure what I do with those vectors (exactly) to rotate them so my projectiles can fire. How would I do this?

Basically, you copy the standard EntityThrowable constructor code, and add or subtract from the player's rotationYaw whatever amount you desire before you calculate the rest of the variables for the heading. Pretty simple, really

Vec3 class has methods for rotation. rotateAroundX(), rotateAroundY(), rotateAroundZ(). You can take the look vector of the entity and do these rotations on that.