No intrinsic reason but on a practical note it might not be compatible with certain legislation around, tax inheritance etc where a single spouse is assumed.
There would be other unanswered questions - if one of them dies does the partnership continue with the remaining two? How would divorce laws work? Could one partner divorce one other or only dissolve the entire partnership.
Sounds like it could keep lawyers in (gainful?) employment for years!
I`ve always favoured two men one woman relationships. It would cut the birth rate and housing shortage. There would be more chance of getting that cupboard fixed and there would always be a babysitter on hand.