how do you calculate the ohm value of a resistor needed to step down the 6 volts to 3 volts?
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Answer it!
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This is only possible if the motor draws a CONSTANT KNOWN amperage. Say for example the motor draws 500milliamperes (that's half an amp or 0.5A.) Ohms law says the resistance would be R=Volts divided by amps.
so 3v divided by 0.5A = 6ohms. So you would need a 6 ohm resistor in series with the motor. HOWEVER, motors use various amounts of amperes depending on the strain on the motor. so you cant really do that with a resistor. However... if you are using a SMALL motor that doesn't use much milliamperes, then you could place a ZENER DIODE in series with the motor. Get a 3.1 volt zener diode. connect it in series with the motor. You must connect the zener diode with proper polarity with respect to the positive battery... One side of the zener diode has a striped end. Connect that striped end to the plus wire. The other end of the zener goes to the plus of the motor. If you connect the zener backwards, then you will only reduce the voltage going to the motor by about 0.7 Volts.
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