I've seen this making the Facebook rounds and I was laughing so hard I was in tears. This is absolutely hysterical. I don't know how ventriloquist do it but this woman, Nina Conti, is carrying on a conversation for 3 people during her Live at the Apollo routine.

Conti picks two people from the audience to participate, a male and a female. She then has them put on these ridiculously oversized mouth pieces that she controls with a string. She then carries the conversation between the two and for herself as well. The two participants basically just stand there, however, they do get into it after a bit and start interacting a little more with movement.