There are 2 issues with that. First, melee breaks autofollow which is used extensively. So hitting moving targets can be a pain, mainly pvp less pve. Second, prot only does damage by getting hit, from both reflective damage and reckonning so having 3 prot is a waste. The only damage your other 2 would be doing would autoattack/seal/judgement/consecration the last two being mana intensive.

If the melee aspect didn't bother you, you could still spec one prot, one ret and one holy and have it work fairly well IMO. Prot for reflective, ret for straight dps and holy for healing/SoR dps(stacking spellpower).