How to power a 1.5V motor with a 3V source?
Motors are basically resistors. While others here will crucify me for saying that, this a basic analogy that's useful for beginners to understand. A motor is a long wire wrapped around a core, that interacts with magnets to spin. All wires have resistance and can be measured like a resistor (use the ohmmeter part of your multimeter).

A basic implementation of ohm law can be used. V = IR.

As you have noticed, the motor is intended for 1.5V to 3V. Motor specs are averages, and rely on specific conditions. Your motor provides X RPM and consumes Y current at Z volts. It will be faster/stronger at 3V than it is at 1.5V, while consuming more current, just like a resistor consumes more current at a higher voltage.

You can use PWM to control the motor, reducing the power compared to the percentage of the PWM signal. Or you can use a resistor to lower the current of the motor (two resistors in series combine the total resistance). Or you can realize that since the motor can accept more or less than its "Typical" voltage, you can give it 3.3V without much issue. It will just be a bit faster and consume a bit more energy (10% more).

Alternatively, you can use a silicon diode in series to drop the voltage 0.7V.