Multiple motor speed control with a single triac
While the answers above are excellent, you have failed to identify the types of AC motors you are using.

Are they brushed motors?  Or squirrel cage types or?

If your motors do not have brushes, then you should not realistically be looking to control speed, as they will naturally try to assume the speed of the windings/armature ratio versus frequency of the supply, and under a real load, you could end up cooking the windings and the triac.  

If your motors have brushes, (ie, like vacuum cleaner motors, blenders, drills and saws use brushed motors), then it's a lot easier, since you're dealing with a commutator that chops the voltage to something like pulsed dc.