I am a proponent of mentally exploring "what if" situations, looking them over thoroughly, & discussing them.
The common goal of a thought experiment is to explore the potential consequences of the principle in question: "A thought experiment is a device with which one performs an intentional, structured process of intellectual deliberation in order to speculate, within a specifiable problem domain, about potential consequents (or antecedents) for a designated antecedent (or consequent)"
At the least, it builds problem-solving skills & encourages deep communication without offering up any actual risk that'd cloud the place up with emotions. It can be both fun & enlightening to play out fantasies.
But I get the impression that the two of you are ready to leap into making this real. That's going to get messy fast.
The script is "marriage+1" & begins from the assumption that someone can simply be stuffed into an extant situation without adversely affecting the initial dyad. And rather than just floating the possibility that
someone might be found who could be properly courted & perhaps eventually
choose to join in, you've already fixated upon a specific individual.
Then, the "escalator" script kicks in: each success demands that the next step MUST be taken. You assume your friend isn't going to run away as soon as the
possibility of nonmonogamy is presented. And then there's woman-on-woman sex, plus the assumption she's at all interested in that with YOU. And then there's three-way sex, plus the assumption she's at all interested in that with YOU. And then there's moving in.
What if she simply stops speaking to you, forever?
What if she's bi but attracted to someone else?
What if the sex isn't particularly great?
What happens if the sex starts out fun but fades out?
What if she is more attracted to one of you than the other?
What if she's already moved in when one (or more) of you decides it's not working out?
What if a Romantic miracle occures & she wants everything your fantasies have already dictated... but she ALSO wants to have sex with other people?
There's plenty of things to think about before you make any changes.