For a 3V motor, connect 3V to the motor power inputs marked "7-12V". Hook motor to A1 and A2.
digitalWrite(4, direction); // LOW for one way, HIGH for the other
analogWrite(5, speed); // 0 for OFF to 255 for full speed.
IF your motor runs on 7 to 12 volts you can close the "VIN" jumper (near the RST pin of the Mini socket) and you can run the Arduino off the same power supply as your motor.
IF your motor uses less than 7 or more than 12 volts you have to cut the "VIN" jumper and provide a separate power supply for the Arduino since the regulator on the Arduino board needs at least 7V and gets too hot if used with more then 12V. You can use motors up to 24V