It's really designed to use with only one mic.
However, you could put 3 mics through a mixer and bus it with one cable to the unit in order to use the effects. You couldn't use the harmonizer though.
If you just need effects there are better and less expensive solutions. If you need a harmonizer, then each band member needs one.
But then again; if you have several band members singing you wouldn't need a harmonizer...
I hope this helps.
Eric