Any chance of some schematics?
Some things to note:
1. The motor is a 3V motor, 4 1.2V batteries is 4.8V.
And the L298 will lose 2.5V or more, its not a great choice for low voltage motors.
2. The stall current is 2.1A - that is how much current is needed to get it moving from stationary
Stall current is the current it takes when the motor is prevented from running (rotor locked in place) and
its given its nominal voltage in full. Its equal to nominal voltage / winding resistance.
The current needed to get it running from stationary should be about the same as the no-load current,
this is due to frictional losses mainly.
(or if under load it depends on the nature of the load).
Is there any load on the motor, or is it just free-running?