I need help designing a speed control dc motor circuit
As its only a 3V motor I'd limit the speed controller range by adding a resistor above and below the pot RV1. A 10k resistor either side will limit the output swing to +/- 5V and give you finer control over the speed. Increasing these values will reduce the voltage range even further if required.

By the way - I suspect your professor was looking for a closed loop type speed control rather than the open loop type you have. Hence the references to ADC, DAC and low pass filters. This would also require a tachometer (usually some form of pulse/frequency device the output of which would require conversion to an analogue signal - DAC) to measure the speed of the motor shaft.
It might be a good idea to clarify from your professor exactly what is required for this project (open loop or closed loop?) before committing to a final design.