I've never played a holy paladin; I made a paladin just because I needed a tank for my first team back in the day, and they're one of two surviving members of that original team. They've only tanked and very limited experiments in ret (even PvP as prot)

I'm going to reply, I guess, to the good and bad I see happening:

GOOD: The shockadin was a viable build at the end of TBC. Then Blizzard changed some things around and made it suck as a side-effect. What I think would be awesome is the rotation of Holy Shock->Exorcism->Word of Glory on whoever needs heals. It would be even better than the old days of running with 4 rets waiting on Art of War procs to make your flash heal instant. Also that conical AoE heal healadins do now (Light of Dawn?) is going to be useful here, as is the other one that heals better if people stay close.(Holy Radiance)

BAD: DPS is going to be an issue here, as are global cooldowns when damage gets a little out of control. Were you talking about having 1 tank and 2 dps DKs? Or were you talking about having 3 tanks? (Spread the love a little) Long cooldowns on holy radiance (down to 30 sec with talents), which isn't really good enough to keep up with the kind of damage you'll be facing in heroics, could be prohibitive. Also you need holy power to run Light of Dawn, although you'll be generating all kinds of holy power just from casting heals through beacon of light (through talents) it'll still take quite a few seconds to generate a really big heal out of it.