Resistance of mini motors

I have 3v mini motors.. i want to connect them to a 12v supply.. so i need to knw hw to find owt the motors resistance so i can connect a resistor in series

Motors have different current draw based on RPM. So first, what RPM do you want to use them at?
Ofcourse this is difficult to tell, so maybe it would be best to do some tests to settle on an rpm you like.

Do you have a PSU where you can vary the voltage? this will help in determining what rpm you want to achieve.

Once you settle on an RPM, you can measure the current draw by the motor. Also check if this current is enough as a startup current. then using the current you can settle on a resistor

The current of a DC motor changes a lot when its load changes. It draws a lot of current when it works hard. Then a series resistor to limit its current will cause it to frequently stall (stopped by the load).
You must provide a constant voltage (with a fairly high current capacity) for the motor to deal with a changing load.

This is only an idea that requires validation by experimentation.

3 volts is 1/4 of 12 volts. So you could drive the motors from 12 volt with a 25% PWM duty cycle.
If the frequency is high enough to allow the motor's inductance to smooth out the current, it could work.