Because of the extra motor controller required, and the increased cost/weight/complexity of a 3 motor gearbox, you would almost have to be in some sort of edge case in order to want to make this trade. That said, I can think of a number of those cases pretty easily:
- Out of CIMs according to rules (most likely useful only if CIMs and Mini-CIMs draw large currents at different times)
- You need a bit of extra torque-to-speed trade space (e.g. for a lifter), but not enough to justify a shifter. This takes advantage of the fact that you can (for example) run 3 miniCIMS at a total of 120A for much longer than you can 2 CIMS without tripping breakers.
- Short depth available behind gearbox