I am gonna stick with the KB45 i think because they are less expensive. And yea i would make a gearbox, not 3 outdrives. Allthough i have seen it done! Even on pontoon boats.
If i did have three outdrives they would all spin a prop at about the same speed the entire time the boat was moving.

I would run the KB45s on 6s most of the time, I am gonna see how one does in my villain first.