Yeah, but if she is open to MMF then it's just one step away from FFM. You can't build Rome in a day.
And seriously, you would be open to a MFF 3P but not open up the FMM door for her? Then you are the roadblock, not her.
You know Sigmund Freud actually believed that all women were bisexual by nature (
http://en.wikipedia.org/wiki/Sigmund_Freud's_views_on_homosexuality). You can see a lot of this in college parties.. a couple of girls down some jello shots and before you know it, they are stripping down and kissing each other.
Your woman would probably laugh in your face if you suggest it straight up, but this is mostly due to conditioning - society doesn't accept it, on a sub-conscious level she realizes she can't have children with a woman, and therefore rejects it. She'll be afraid you'll fall in love with the other girl and leave her, what would her parents or friends think?
There are a host of reasons.. but if you can build her trust in you, show her that being bi-sexually is commonly accepted in society now days.. she'll open up to it faster than you can imagine.
Of course, some women are really repulsed by it
(that I've seen) and you probably can't win there.