To learn about the gearboxes and other mechanical things, I suggest you look at 973's RAMP videos:
https://www.youtube.com/user/973RAMP. I'm not sure there's anything specific on gearbox design for drivetrains, but all of the instructional videos are incredibly informational.
Assuming that you used a kit-bot frame this year, the easiest upgrade to 3 CIMs is
this. Otherwise, there are quite a number of different COTS gearboxes that can be bought from either andymark or vexPro.