When you have a threesome and two of the three are related.(one guy fucks two cousins, a brother and a sister fuck a friend, mom and son fuck friend, ect)
Hey man how'd y'all go last night?
It was great we had a southrn style 3sum
Nice brotha who wer they?
It was me my ma and this girl I met.
Incest is best brotha.
God bless Texas
A wholesome mix of homosexuality and heterosexuality. Or you can just say, from the movie Get Him to the Greek, "It's just like having sex with your girlfriend while someone else also has sex with your girlfriend."