The math is simple......

To run a same performance you need to have a same power. With a 2S to 3S battery that is a 1.5 factor with the voltage. Because power = made by voltage x current so you do need a 1.5x lower current, and to get the same runtime also a 1.5x lower capacity. To be honest, a 3S hardcased battery can just store a 1.5x smaller capacity.

Then there are other factors. Yes, on 3S you have lower currents which gives less loss on wires, solder joints and connectors.but the 3rd cell has a higher battery resistance. So at the end the efficiency of the motor, ESC and maybe battery will make the difference.