It's a trap !!! Don't do it!!! I'd forget about the fantasy for the time being.
How I personally interpret the whole thing...
I'm guessing your wife isn't putting out, or if she is it's just token. S ex, which is ****. You probably feel like she's putting in no effort , or passion into the relationship and she probably isn't.
Which is frustrating as ****, and probably not your fault.
Clearly she's not meeting your needs right now.
She's probably feeling ****ty about herself, feeling unattractive, secretly resenting you because she believes she should be getting all that from you.
Blaming you because she's not feeling sexy, fulfilled.
Probably taking ZERO accountability, and she probably has little to no inclination to improve her situation or your marriage.
Hence the ""free pass" to go sleep with other women. "If you think I'm so **** go get your needs met by other women you **** " .
Seems to me like it's a deceleration of war, you do it and it's where things get real ugly.
You will 100% become the bast ard, and she will make your life difficult.
She's also probably on the path to cheating herself. She doesn't feel good about herself, she probably not taking accountability for that and will just try find a guy to make her feel good about herself.
How I'd attempt to handle it...
I'd slap a band aid on right now. Just plan something nice one evening, address the threesome, something along the lines , "her mentioning the threesome with other women, only made you realise how important she is to you and it's her you want " , remind her why you married her. Blah blah blah.
Just advert the immediate crisis. No threesome talk or talk of sex outside the marriage for the immediate future.
Then focus on you. Looks like a sinking ship. Regardless of you want to save the marriage or bail. The path is still the same.
Work on you. Picture the guy you want to be killing it when you're single, get all ALL your **** together.
Genuinely improve to be that guy. There MUST be things you don't have a handle on right now. Work on yourself.
Stop the threesome talk, stop accepting token sex, take pressure off her. She's no longer relevant.
Improve you.
She's your wife so DO make time to have awesome times with her,, do romantic things, only accept great and genuine sex e.t.c.
Just don't accept the ****ty stuff she's bringing to the table. Focus on you.
She will probably see the path you're on. Mabye fall in line, mabye not.
Do this for a good while.
Mabye your marriage will feel fulfilling again and you won't even want a threesome ?
Mabye she will begin seeing how awesome you are again and be so into you, she will feel so sexual around you that she will want to have that threesome and actively seek it to please you?
Mabye things will stay the same with her, but that's good enough not to leave , And you decide you want that threesome, so you just cheat (do it behind her back).
Mabye she will just stay the same and you will just need to walk away.
Doesn't look like you're ready to bail on her just yet. So I'm suggesting just work on yourself and stack the deck in your favour regardless of outcome.
Path is still the same in my opinion.
Let her feel genuine dread. Let her see the path your on, actively see you becoming her best possible you. She either steps up or she doesn't.
I honestly hope. Things turn around for you and she becomes so sexual and into you that she seeks out that threesome from the right place.
BUT The way she's presenting it right is FAR from the right place.