Jon --
welcome to our world!
Most people I talk to say its taken
Y E A R S to convince their better half (female) to engage in a MFM threesome.
It is my experience that the turning point is when the wife/gf discovers that
SEX=LOVE isn't the only formula ...
SEX=ENTERTAINMENT SEX=ADVENTURE SEX=INTELLECTUAL STIMULATION
are just some of the equations that, once they discover work well for them, gives them an entirely new perspective on screwing
if your GF is sexually liberated, maybe she already appreciates the other equations. but if she's still focused on SEX=LOVE, then you've got some work ahead of you.
if i was starting over again, i would concentrate on discovering which formula she has in her head, and then introducing her to the others is a safe environment (videos, 'pillow talk', etc.) you'll know when she's ready
good luck.
Mr. S.H.