Karelia, dunno if this will help, but I'm currently in a vee. I started off dating my fiancÚ, and have recently begun dating another woman I have been friends with for a while. (I guess it's not technically a true vee because I don't rule out he possibility that I might date others, and my fiancÚ and girlfriend are also both free to do the same. None of us is seeing anyone else yet.) It does present some challenges... My fiancÚ was initially hoping we could have a true triad, but my girlfriend is a lesbian and doesn't really have interest in dating men, even though my fiancÚ is only male in form. He's very feminine, and his femininity is one of the aspects of his personality I love most of all. Still, it was a no-go for my girlfriend, and she learned this after we had a "test the waters" threesome. We've since had another threesome, but my fiancÚ and girlfriend didn't interact much.
Anyhow, I can imagine there would be some awkwardness at first, and I've thought it would be easier in my own situation if my fiancÚ and girlfriend could also date each other, but I'd imagine that would present its own complications, too. Every relationship is complicated, but I think it would be helpful to clearly outline your expectations and see if it's something the three of you can live with. Reassure your husband you aren't going to conspire against him and leave him, and tell your girlfriend you have no interest in ending your relationship just because your husband's feelings have changed. There's probably a fine way to make this all work, don't fret just because it's not the same as it's always been!
Good luck to you three!