install a dps meter, recount/skada. use the treants, and if we say that they do 3000 damage combined you do 3000/3= 1000 so every treant does 1000 damage
if you want more detail you can see how many times the have hit their target (let's say 30) divide it with 3 then have have hit the target 10 times each.
the restoO treant dont hit,
the balance treant hit with spells,
the feral treeant hits with melee,
the gaurdian treeants hit with melee but do low dammage
also i am quite sure they scale of your gear, else this talent becomes useless in end game gear