I am in a unique situation.

Here's the background context:

I am in a relationship with a beautiful, intelligent woman. She is 25 and blazingly smart, academically and otherwise. I developed a friends-with-benefits relationship with her at first, spurred by our mutual desire to explore new sexual territory. One sexual fantasy we wanted to entertain was a threesome. It happened, and then it happened again, and well, now we're dating.

One thing about us was that, originally, we wanted to try a polyamourous rellationship. We wanted to find a third. The first girl, let's call her Samantha, we both really liked her. Unfortunately, she didn't live in the same city.

Well, things have gotten interesting. Samantha has independently expressed interest in a three-way relationship with us. She's moving back to where we live, and she wants us all to be together.

Here's the issue:

I love my girlfriend. My girlfriend loves me. Samantha loves me. Samantha loves my girlfriend. However, my girlfriend doesn't love Samantha. She's expressed that she is sexually attracted to women, but not emotionally.

I would love nothing more than for a polyamourous relationship. The hurdle is in motovating my girlfriend to not only be interested in Samantha sexually, but emotionally as well. She needs to feel like it is her own decision. What can I do? I need help in this regard.

Help me out, here. I can only plant so many seeds, but I really need advice on how to groom the plants that grow.