@TinkyWinkyWonky Because you have doubts now, you definitely should not do this.
However, I think that you want to do this for both yourself and your partner? If you want your present relationship to be a forever one, I will again say, please do not do this. I have known - over many years - a couple of couples who have done this, and at the time they both seemed to really enjoy it. But 10 to 15 years later it was a different story. The man of the couple in both couples still looks back at that time very fondly! Unfortunately, both women (they don't know each other) have subsequently told me that although they enjoyed it at the time, it now makes them feel sick through late developed jealousy, as they are still with the same partner as they were at the time. I am no expert OP, but I imagine that if they had had a threesome with a partner they were never that serious about, then maybe it would still be an okay memory?
Sadly, I think that you are not going to listen to the majority of us who think that that is a very dangerous game to play, especially if you are at all invested in your present relationship. So, I am going to suggest a few rules if you insist on going ahead with this. But I must admit to being curious about how you and your partner intend to find a reasonably attractive, healthy - particularly considering her sexual health - woman, who is willing to have a one night stand with you both, and to never see either of you, or contact either of you, again? Because I hoped to give you this advise before you leave the thread, I have only read your OP so far, so you may well have already addressed this 'sticky' problem already!
So, here are my suggested, potential, rules, if you still insist on going ahead:
You really must not have the threesome in your own home, in fact you mustn't have it in your home town, or even your nearest city. Plan a few days away at a hotel somewhere - preferably in a different part of the country altogether - and plan to have the threesome on your last night there. It is also very important that neither of you ever know the other woman's address.
Even with the above safeguards that I have mentioned, I still think it is a very bad idea, and it will still have the strong potential to ruin your relationship in the longer term. Please never include someone either of you already know, in any capacity whatsoever. You can still have fabulous fantasies without involving anyone else, maybe one could be including a weekend away in a hotel, where you pretend to meet in the bar as strangers?