You want to know how to have a threesome with your wife.
This is a common fetish.
Lots of men approach their wives in the hopes they will agree to a ménage à trois.
The subject of introducing a sexual threesome into your relationship should only ever be broached when you and your wife are in a sexually aroused state.
This is not a subject that can be discussed while getting dressed for work.
Your wife needs to be in the right mood.
Be entirely open with her and tell her about your fantasy.
Make it abundantly clear that, should you want to include another woman, it has nothing to do with your sexual needs being unmet by your wife.
Be clear that you are simply aroused by the idea of you and your wife having sex with another person.
You can suggest to your wife, if this is a new idea in your relationship, that you could roleplay a threesome next time you make love.
Pretending is easier than the real thing.
It will also ease you into doing it in real life.
By this stage, if your wife is ever going to want a threesome, she will want it by now.
Lots of single people are, just like you, aroused by the idea of a ménage à trois.
You can find them on most adult dating sites by searching for single men or women who are looking for couples.
Be sure to get to know the person before you meet.
The ideal candidate is someone who will not get emotionally attached to you or your wife.
This is how marriages get destroyed.
The best person to find is commonly single men or women who are not interested in relationships and just want some fun.