mamba motor shaft pinion size?

BL is definitely correct! A 5700 on 3s would be ridiculous. I ran a 5700 on a 4200mah 2s geared 13/46 and it had way more top speed than necessary for racing. I'd recommend either 2s or a motor with a kv around or slightly over 3000 for the 3s. A standard size motor works alright but runs a tad on the warm side. An "L" motor on 3s is actually alot better suited to the size and wieght of the truck.