Voltage reduction issues in an existing circuit
I'm guessing at your schematic, but if I assume you simply place the resistor in series with the motor then a quick calculation shows that your resistor is severely underrated:

To drop the voltage from 7.4 to 3 volt there must be 4.4 volts across the resistor. Using the formula P = U² / R it follows that P = 3.9 Watt, way too much for your 5 ohm, 1/2 Watt resistor. It may be okay for short bursts (e.g. trigger pulls) but for repeated / long press it will overheat.

Also note that the DC resistance you measure across your motor with a multimeter is useless. When a motor runs it draws less current due to back EMK. The only proper way is to measure the current while the motor is running. You will also find that there is a non-linear relationship between voltage and current; so you have to experiment to find the proper value.