I doubt it's logistically possible to give 3 quarterbacks enough reps to adequately prepare them for games and to compete for the top job. It's hard to do it with 2, and the delay in figuring out the starting quarterback is going to show when the season starts. An NFL quarterback needs lots of reps with his team to truly be effective. Splitting practices, which are already strictly limited, into 3 groupings, would be irresponsible.