Why you want a third diff? I don't think the drivetrain will last long and wear out rapidly or destroys it. 1/8 nitro buggy uses a third gear but that is because they use a clutchbell which acts the same way a slipperclutch. Therefor major brands like Kyosho, PR-Racing still uses a slipper clutch in their 10th scale buggy's while Team Associated uses a fluid filled 3rd geardiff. Both have their pros and cons but in my design I have chosen to use a slipper to protect the drivetrain. Maybe in the near future Ill look for a possibilty to mount a third diff but it will not be a gear diff. Maybe a balldiff because that also has some slip for a the massive power of todays brushless motors.