Um... I didn't actually look stuff up, that was just my observation. I've seen it proc a lot so far and never seen three hits with high damage, always only two. Does it hide the third hit? I know I read somewhere it's only 2/3 hits, now I looked on wiki people saying 3/3 hits. I've never seen this happen before myself. Is that related to the missing offhand hit?
It doesn't matter, it's still only 35% activation rate. Whether looking at each hand, or combined, only thing that matters is the number of total hits.
Assuming 100 total hits (single): 35%*100hits = 35hits
Assuming 50 hits with each hand: 35%*50hits + 35%*50hits = 35%*100hits = 35hits
So the same number of hits are candidates for the Triple Attack augment.
I still wanna recalculate with 3/3 hit activation (300% damage, so 200% bonus):
1st option:
108hits + 1.89hits*200% = 111.8hits
2nd option:
100hits + 3.5hits*200% = 107hits
So only 4.8% better instead of 5.5%, and still not accounting for any haste and other stuff, that would all count towards the 1st option without full AF3+2.