I have a hunter on my alt account I use to power level characters on my primary; having moved said hunter from my primary long ago

As such I used my Hunter in a background session. This was before the dead-zone fix in 2.3

Shot rotation was very easy to maintain. For the most part I used him with my smite priest and elemental shaman. With two characters you don't need an aggressive shot rotation, for the most part steady shot threading is all you need. Now if you were doing five man and aiming for instance work I would work in arcane shot and the like (perhaps even for arena teams)

What are you going to gain with three level 70s other than Arena and possibly BG (provided all enter the same BG!). I see three man as a leveling setup, two man is easier for farming or such.