For a 3A motor, you would be much better off using a logic level N-channel mosfet instead of a transistor. It will run much cooler and you won't have a problem providing enough base current. In that circuit, replace T1 by a logic level mosfet, and replace the 3k3 resistor by one in range 100 to 220 ohms.
My rule of thumb:
- to switch up to 500mA, use a transistor
- to switch more than 500mA, use a mosfet
- never use a darlington
PS - that motor has a current drain of 21A @ 7.2v when stalled. So unless you implement some form of current limiting, the mosfet needs to be rated at 21A or more. The diode in parallel with the motor similarly needs a peak (not necessarily continuous) rating of 21A.