A three person couple.
On Big Love, they were a thrupple until Ginnifer Goodwin's character came around.
Alternative term for menage a trois, for those of us who are less talented in the correct pronunciations of foreign words. Derived from the words threesome and couple.
'Did you make a decision on whether you were going to carry on seeing the girl with the big tits or the one with the great ass?'
'No, I couldn't decide but I did manage to persuade them both to engage in a thrupple with me.'