I didn't think to buy three. Then again, I don't see a 3-pack available. So I have the 3 which came with the unit, and the 4 Powerex I purchased, which I plan to swap back & forth. I'd have to buy 2 more sets of 4 Powerex to get me to 12 (and thus a factor of 3), which would be a total of 15 batteries, which is a bit much.
I guess I will mark them 1-4 and rotate them. I have the C9000, which takes 1-4. It's a terrific charger.