If you only supply 1.5A to a 3A motor, you'll only get half the torque. Torque is proportional to the current supplied.
Typically, you'll want to supply a stepper with 10-20x the motors rated voltage. A stepper's torque falls off quickly as rpm's increase. Higher voltages allow current to flow through the motor faster, maintaing torque at higher rpm's. Doubling the voltage will usually double your useable top speed. Too much voltage, however, will cause excessive motor heating, so there is a limit to how high you can safely go. Usually around 20-25x the rated voltage.