I’m confused—as far as I can tell, the three motors are powering the three gears in the back. Problem is, those gears have 84, 36, and 84 teeth (or at least they look like that to me). Fixing the motor speeds in code is the only way for this drivetrain to work. Even then, the drivetrain is suboptimal.
Suggestion:
Move the 2 frontmost motors up to the gears second from the front. Add a small gears in the back directly above the already existing small gears (although this might interfere with your vertical C-channels). Move the 2 rearmost motors up to the newly added small gears. For reference:
Although you could just build a drivetrain with another gear ratio. Most of the gear ratios in the Catalogue of Drive Gearings are okay. If you only have 4" wheels, 36 tooth gears, and 84 tooth gears, the drivetrain pictured above is basically your only option (although you could just use four motors instead of six).
Literally me:
Also, sry for the long posts, I kinda suck at writing. (Why are my posts annoyingly long sometimes but my writing for school annoyingly short always?)