Think it depends on the system you run. You dont need to take a QB every year if you run a pro system if you have a couple guys you feel good about you can manage with a smaller room with 3 or 4 guys. If you want mobile QBs you should probably take one every year. Mobile QB's put themselves at risk for injury more often and they can often play multiple positions if QB doesnt work out. We all know your stance on the QB room as it is discussed on here often.
3 is not ideal for a offense like we are looking to implement, but you can take care of your QB's by how you call plays and teaching the not to finish runs with contact but look for the sideline or the ground.
QBs transfer so often, it's best to take 1 per year regardless of system.