My team has used the three-chain design for two years now, to great success. Though we have never used the kitbot transmissions.
I can't recall the exact shaft size we used for our axles, but I do remember that, two years ago, by the time we were half way through the qualifying rounds, the bolts we used were toast. We upgraded to some grade 8 bolts of the same size and haven't had a problem since. Though, getting those bent axles out of our wheels was a bit of a challenge I must admit. You can see a picture of the design we used two years ago
here. I couldn't find much of a shot of the drive train from last year.
Other than the bent axles, the only trouble we ever had was getting those poof-balls stuck in the chains. We actually cut one in half during a match once, that was pretty cool... well, until it finally popped one of the chains off anyways. *Hopefully* we won't have a problem with that this year though...