"I will agree with Earthdog that some FITA coaches only knows that type of shooting (I know some of them) but the basic FITA form is sound."
3 under is not common in FITA.
I agree though that if you do not have faith in your coach, then I would suggest finding another. It sounds like your current coach is using a "command" style of teaching where you might fit better with a "cooperative" or "submissive" (although I do not care for this one) style. Better coaches will know these styles and are usually able to coach in either type. I'd talk to current coach first before releaing him and if he still only wants it his way, then mutually agree to part ways.
Personally, depending on goals, 3 under or split are acceptable. If a person if having trouble keeping the arrow on the rest, then I usually suggest moving to 3 under. Some love it and stick with it especially if they shoot gap.