3 VDC Motor

Something like this?[PDF] 500 mA, Low Voltage, Low Quiescent LDO Regulator - Microchip

Or, less sophisticated:Potentiometers In Stock - At Mouser An Authorized Distributor

Power= Volt x Amp, 3V x 0.5A= 1.5 Watts, use the sites above and get a wire wound potentiometer rated at 2 Watts or more, I'll leave the resistance range as an exercise for you.

what sites above ?

The speed control of DC motors is not done by varying the dc supplied but it is done by using PWM of the supply(3V). This is done as the motor will not have torque at low speeds as well as it will not start to rotate with low voltage.

Gajanan Phadte

The speed control of dc motors is often done by varying the voltage. Whether or not it works depends entirely on the speed range you need (you can get up to 10:1 with voltage control, which is often enough, vs. more than 100:1 with PWM) and the motor loading, and particularly how the load changes with speed. On many applications the required torque at reduced speed drops more than the motor torque at reduced voltage. The OP doesn't state the application, nor the speed range required, so we don't know if PWM is required or not.

The speed control of DC motors is not done by varying the dc supplied but it is done by using PWM of the supply(3V).
In most cases PWM acts as the means through which voltage is varied. Many DC motors (and especially 3 volt motors which are virtually always permanent magnet type) have an rpm/volt constant in recognition of the fact that speed varies with voltage, and torque varies with current.
Varying voltage with a large pot is inefficient, but for the OP's purposes it might work fine.