Well, you'd have all of the Pally buffs you could want.
The Prot pally needs to be hit to function, but with only one that's fine.
One of the Ret pallies would need to have a fair bit of Holy, as you progressed to more difficult content.

Three Ret pallies provides decent damage and a lot of utility.
You can round-robin cast a Hand of Protection, by assisting the active tank... something like "/castsequence [help] Hand of Protection,,; [help, target=targettarget] Hand of Protection,,". Preface the castsequence with the assist method of choice to target your active toon. So any toon takes aggro from the tank, and the group has 5x BoP which should put aggro firmly back upon the tank.

I don't like multiple melee as your dps, but for PvE this is much less of an issue then it is for PvP.
A holy pally is an excellent single target healer, but can be lacking for healing AoE damage.

IMO, a PvE group should be diverse in desired gear.
As in, Plate x1, Chain x1, Leather x1, Cloth x1, and one duplicate.
With all your toons capable of wearing plate, you could equip them in whatever dropped, so gear won't necessarily go to waste.

I'd recommend Enchanting and Skinning on a secondary toon, and Mining/Herbalism on the usual leader toon. You can macro skinning, via a targetlastenemy macro, but mining/herbalism you need to actually click the resource. Enchanting will ensure your BoP drops aren't wasted if no one is using them. Engineering is a good option for your active toon, Pallies are somewhat lacking in the ranged damage department.

All in all, not necessarily the best composition, but definitely workable if you want to do it.
Bottom line, will this be fun for you?