Resistor to limit maximum voltage of solar panels
First off, you can't connect unmatched panels in series. Why? Because power will be dissipated in the unmatched module. If three modules are connected in series, they need to be the same.

Secondly, if you don't have an MPPT tracker, your not using the full power of the modules. An MPPT tracker will most likely solve your problems. Why? Because the motor may not be drawing the right amount of current from the modules and if the right amount of current is not drawn, the power capability goes down.

Thirdly, V=I*R is the best way to size resistors.