don't. I think it's a bad idea because it will introduce feelings of intimacy for partners outside your relationship and jealousy.
You only live once comes to mind! or even don't knock it until you've tried it. Sexuality overcomes most negative thoughts about this type of relationship. That is if the "third" is good at providing an extra dimension to the relationship.
If she doesn't want to do not pressure her. If you want it to happen and be a good experience she has to decide she wants it on her own. Instead of asking her to do it persuade her, maybe by showing her pictures of men she finds attractive, sharing erotic stories from sites like literotica, let her "find" your internet searches or pictures of erotic threesomes. But be careful, as the saying goes, for you may get what you wish for and come to regret it...
We started out reading the stories in Penthouse! Now I have written several blogs about our experiences! Check them out on here! ❤! Alice
Wile it sounds like twice the fun I had two women in bed once and what happened was wile they were both attractive I was more attracted to one then the other and this created an awkward situation because I did not want anyone feeling bad for getting less attention.