Maintaining a current in a circuit connected to multiple DC motors (3.0V) that turn on and off arbitrarily
Measure the DC resistance, R, of one of your motors.
Calculate the starting current using \$ I = \frac V R \$ where V is the supply voltage.
Your power supply current rating should be ≥ \$In\$ where \$I\$ is the calculated current above and \$n\$ is the maximum number of motors to be run at any time.