But somebody said must be same layer at both side, since I'd check two layers is Max. for bottom side, cause three layers will touch to the Motor.
there's no such thing as same layers on both sides, u can put no FETs on one side and the car would still work.
but u need to bear in mind the rule of "diminishing returns"
usually the increase in so called 'performance' becomes insignificant after the 3rd layer.