Hi, Seyscouple and welcome.
You might want to search here for 'unicorns.' That's the poly slang for what you're looking for and, as the name implies, it's very hard to find. For one thing, it's very hard to fit into an existing couple's structure. I'm not saying that it doesn't happen, just that the women who are willing to do this are few and far between. I've been asked and I wanted to be the unicorn, but it has never worked out for me. I know many women for whom this is true. It might be worth discussing whether or not you are willing to go in whatever direction life and love takes you rather than trying to shape it to your will.
And I echo AT. It's not a lifestyle, it's a life.