Three phase motors are generally regarded as more efficient than single phase motors, but you have the inefficiency of the phase converter to consider, which depending upon type is either a mess of power electronics (static phase converter) or essentially a single phase motor/three phase generator rolled into a single rotating machine (rotary phase converter). You can even use a plain ordinary three phase induction motor as a rotary phase converter; you power it from your single phase supply through two of the leads, do something mechanical to start it spinning (since a single phased 3 phase motor won't _start_), and once it is spinning you can draw the third leg from the motor terminal.
For air handling equipment, rather than using a phase converter, it is worth while considering using a variable speed drives. Air handling is the classic case where variable speed can save energy. For a given duct system, the power required to move air scales roughly as the cube of air speed, but the volume of air moved scales linearly with the air speed. Rather then running your motors at full speed, and then throttling the airflow or turning the system on and off, operating at low speed continuously with minimum resistance to air flow can save lots of energy.
-Jon
