Well, I was helpin a guildie get his tank geared so he could run his own 5 box heroics, so we were basically running 2 DPS 1 healer 1 tank and 1 offtank, so the DPS was equivalent to just having 4 toons. It was pretty easy actually. I think that if you get one DPS that can pump out well over 3k DPS, like 5-6k DPS on boss fights, that you could 3 box heroics, provided your tank and healer are well geared. Only trouble with all of this is that if you overgear them that much, the only reason to do em is to farm badges, and if you have enchanting, u can make good money off shards (dream shards on my server regularly sell for 350G a stack of 20) If you could swing it, 4 would be pretty nice. My locks are hardly overgeared, but they do quite good DPS.

So if I were you, I'd roll up a lock and a shaman, lock for good DPS with so so gear, shaman for the same, as well as bloodlust/heroism, and totems. both of those toons can do 3k DPS easily in heroics. Holy pally isn't the ideal healer for heroics, but as long as you put beacon on the tank, quasi AOE healing is doable.