As far as I can know, to control the speed of a three phase motor, you need to change the frequency of the "AC" output. To control the speed of a brushed motor you need to change the voltage, unless you raise the frequency to a value where the iron losses in the motor become excessive.
There is also a slight problem with three phases with one motor, and one with the other - current limit. Depending on what you actually want to do and the range of control, you could feed the three phase into bridge rectifiers giving pulsing DC then feed this via an inductor to give an inverse frequency/voltage output relationship.
Frank