hi,
my team and I wanted to build a gear for a drivetrain that uses 2 CIM motors and one mini CIM
is this legit or it wont work well?
Assuming you mean gearbox instead of gear (English is tricky 
) then yes that will work. miniCIMs were designed to be a drop in replacement for a CIM with about 2/3 of the power. That means any gearbox designed to work with 3 CIMs will also work with 2 CIMs and 1 miniCIM.
Yes, it will work OK. See this thread for some applicable calculations. It starts with 4 vs 6 CIMs but adds mini-CIMs in as the thread spins.
That sounds like a good plan! If you use one of VexPro’s 3 CIM ball shifter gearboxes, you can replace one of the 3 slots with a Mini-CIM if you don’t need the power of a 6CIM drive or are using the CIMs elsewhere. I would also recommend considering the use of 775pros anywhere you can as they have a much higher power to weight ratio than a CIM, and you can then have a full 6-CIM drivetrain if that is what you want.
I know this is not what was being said above, but just to be clear given where this thread started: Do NOT mix CIMs and 775pros at the same gearing, and do not replace a CIM with a 775pro without changing gear ratios! The 775pro runs about 3.5x as fast as the CIM.