I guess part of me is thinking if I see them intimate with each other it would help alleviate some of my anxiety about it. If I share in that intimacy with them then I may strip the power away from some of my fear.
The thought of fulfilling one of her biggest fantasies is extremely arousing to me...we've even fantasized about it verbally while I pleased her.
While it could happen this way, it is INCREDIBLY unlikely to. Threesomes, and triads, almost never allay insecurity or jealousy--they magnify it. This is going to be graphic:
-What if his penis is bigger? What if it bends differently and seems to please her more? What if her orgasms look and sound different with him than they do with you (hint: they do, I promise)? What if they seem like they're better?! What if they kiss differently (again, they do), and what if it seems like she's enjoying it more? What if he does something with/to him she didn't ever do with you? You get the idea. What if you cum first, and they want to keep going? What if he gets more penetration time/oral sex/whatever? Or she is more (seemingly) excited for that with him than you?
And that is just the beginning. I've been in threesomes. A lot of them. And, even with no cheating and no jealousy, even with no trust issues, they can still go really sideways.
And, again, I am going to suggest you think about the possibility that all this is hysterical bonding. I've been through that, too, and it is crazy-making, passionate, and all kinds of messed up in terms of what sounds like a good idea. Essentially, it's an intoxicating cocktail of hormones and emotions, and is pretty well understood psychologically. It is what makes couples think it'll all be so okay, so fast, and that everything is fine just weeks after an affair. It masks the issues with a desperate need to physically reconnect. it's common to see a huge spike in sex drive and sexual activity, adventurousness and new sexual acts, sexual emotions that may not have been felt before, and an over all lowering of inhibitions. Think of it as one of the stages of grief--it's definitely in the same ballpark.
For the healing couple, this can be a good thing. It can be a time to reconnect and rekindle the flame of passion. But, it will end, and the trust and communication will still be broken. And, if you do something like a threesome, it can kill those hormones really, really fast (generally in the middle of it, and I am not even sure if you can imagine how badly that can go), and further damage the relationship as the reality of pain and betrayal comes crashing back in.
You two have not healed.
You have not healed. In two months, it is very likely you will be in a very different place. Poly? Possibly. That would be great, and possibly the best outcome for all involved. But, very possibly not.
I am not trying to be a downer. And I get that you want to do this right now, and that it seems really urgent, and that you want to believe your relationship will heal differently, that you will heal differently, than the millions of people who go through infidelity. And, there is a really tiny chance you might, but, again, it's really unlikely. The stages of grief, which apply to cheating as much as death, are pretty established for a reason. The far more likely scenario is that you will, at some point, come down from this "high," and that crash is going to be incredibly hard if you and she don't work on the actual things that will heal you, rather than the band-aids that seem like a fast, hot fix.
Fantasies are great (aside: when one of my LTR ex's cheated on me, the sex life and fantasies got CRAZY--and wow, do I regret every fucking minute of that now). Keep this in fantasy world for at least 6 months. Read up on healing from cheating, rebuilding trust, honest communication, etc. Do the footwork first, the threesome later.
