Your driver TB6600 is supplied with a higher than 3 volts voltage.
The driver will supply that voltage to the coil of the motor until it reaches the setup current. The higher the voltage, the faster it reaches the setup current. At that moment it will switch of. The current go's down, and it will switch on again until the setup current is reached. this continues.
This is because it's a coil in the motor, not just a resistor. In a ideal coil the current increases linear with the time.
If your motor is comfortably warm after 5 to 10 minutes running, the current is fine.
Normally the setup current is for one coil