When using the same motor and all around setup, going to a higher voltage will keep the same runtime but be faster. Actually, it may be a little higher because you won't be into the throttle as much.
The
parameter solver is perfect for "what-if" scenarios like this. Just enter all your current data for the 2s setup, except the speed. The answer will be the speed. Then, clear everything and enter everything the same except change 2s to 3s and leave the pinion entry black. The answer will be the pinion you need.
Since you are keeping the same overall speed, runtime will be about the same since the motor is doing the same amount of work.