A great rock song to dance to is "Kick Out the Jams" by the MC5 (also covered by Rage Against the Machine). I don't know whether you'll be allowed to swear, so you might have to cut out the cry of, "Kick out the jams, motherf***ers!" at the beginning.

I seriously suggest considering an ABBA cover! They're easy songs, great to dance to; and you two are funny guys who ought to be able to get the crowd going. If it's just the three of you, "Mamma Mia" may not be a good idea; perhaps "Dancing Queen"?

Other good ones:
#"Roll Over DJ" and/or "Are You Gonna Be My Girl" by Jet (the ladies love this band)
#"Ballroom Blitz" by Sweet (disco classic, plus the comedy factor)
#"You Shook Me All Night Long" by AC/DC
#"I Wanna Rock and Roll All Night" by KISS
#"The Hand that Feeds" by Nine Inch Nails (easy to play, ought to please the goths and the techno fans, and it'll remind the crowd that you're still living in the same decade as them if you've been playing any seventies songs)
# "War Pigs" by Black Sabbath (maybe too long a song for a disco gig, but I was just thinking that it's manageable for a guitar-drums-bass trio, it rocks, it's danceable, and it gives you all the opportunity to show off with the drum fills and guitar solos...)