You need a low-pass filter (R-C circuit) to "smooth out" the PWM signal into an analogue signal. Then use a simple voltage divider to produce the 3 volt maximum signal level required. Note that the R component of the filter will "lose" some of the 5 volts and the C component will cause a delay in the circuit responding to a change in speed request. If you chose relatively low values of resistors in your potential divider (say in the order of a 10k chain) then the delay effect should be minimal.