The problem will be getting geared enough to do the heroics 3 box. You'll likely have to pug your way through them just to get the gear so you can do it yourself. But by then you won't be getting anything useful out of them except some emblems.

If you absolutely can't get 5 accts up try making some friends to do the heroics with you or join a guild. Once you know the instances and people realize you make it easy they will start asking to go with you. No one likes risking the random pugs where you never know how many wipes you will get.

I don't doubt every single heroic can be 3 boxed if you overgear it. The hardest one would probably be occulus because of the last boss. My guess on the easiest would be Utgarde Keep. Nothing in there is very anti-low dps. You can take your time and as long as you don't run out of mana you should be fine.