Initially I tried it just as you describe (standard unit made up of three 60x30 bases), as per the unit frontages table, but I found movement around the board/field to be a nuisance. Also, my collection was naturally much smaller when I first started playing, and so I found that reducing the standard unit size to single 60x30 bases was more conducive to the way I wanted to play. I've found that HC accommodates 6mm quite well, so long as you're willing to make a few minor adjustments to unit sizes, movement distance, range, etc. 

And yes: I use 3 divisions per belligerent in my bigger battles and 2 with skirmishes.

Cheers!