Playing unplugged as a soloist isn't that difficult, but in this setting, I couldn't hear Paul's guitar, which led to some flubbed rhythms. It was even worse whenever I turned to make eye contact with the extra audience on my side of the room.
I think that's the reason Peter, Paul & Mary always stood around a single mic. Of course, Mary didn't play guitar, so that made the one mic thing easier. Harder to fit three guitarists around one mic.