Ah, I see from post #9 that your PWM rate is only 695 Hz. Just increase the value of C4 to at least 4.7μF and my circuit should work ok with your existing PWM as the input signal.
If you think 40kHz is too high a rate you can reduce it by increasing R1 and R3. R3 may need adjusting anyway if Q2's gain is unusually high or low, so best make R3 a 10k trimmer.
Note that only 2 gates of the CD40106 are used per motor. So the one IC will handle 3 motors.
For anyone wanting an explanation of the circuit:
R4,C4 smooth the input voltage at Q2 base.
U1a, R1, C1 form a 40kHz oscillator.
Q2,R3 form a constant-current source controlled by the smoothed input voltage.
At the beginning of each oscillator cycle R2,C2 provide a brief pulse to turn on Q1 and so discharge C3.
U1b output goes high in response.
C3 then charges up from the current source.
When the C3 voltage reaches the upper Schmitt-trigger threshold of U1b the output of U1b goes low.
The resulting 'high' time of U1b output in each cycle, i.e. the duty cycle, is thus controlled by the Q2 base voltage.