I find the question begging the definition of marriage, and the legal ramifications. If the three people wish to be together, so be it, but legally they will not, and i believe they should not, receive all the societal rewards of "marriage." Any two people who wish to enter "marriage" should be able to do so and derive all the legal privileges and protections. Another person entering that relationship would not. This would make for a very unequal partnership, not at all what marriage is intended to be (we humans screw it up royally, I know)
Barbara