I don't want to sound selfish, but we know what we want, and we hope to find a single person to add to our marriage, not a couple. We kind of don't think that will be much of an issue simply because we won't be looking for that couple, but be seeking out a single who hopefully will be seeking for a couple like us.
My guess is that this will be greatly limit your options of partners, especially considering that you're not (I assume?) in the age bracket where people are not yet likely to have sought out a life partnership (late teens, early 20s). There are definitely some great solo poly people out there who are older than that (hi Nycindie!
), but it may well be that they are single out of choice and aren't looking for a closed life partnership.
I think it's this desire that the new person come without existing partnerships or existing "complications" that leads to unicorns often being younger then the couple they're joining (definitely a pattern I've noticed, at least). This can lead to other problems, unfortunately, such as the new person being a little naive and not really knowing what they want or what they're getting into, and/or not having the communication and relationship skills to navigate the situation once they're in it.
It takes someone really strong and confident to stand up for their needs in a healthy, effective way in the face of two people with a long-standing partnership when they're the newbie who's still figuring out how they fit in, *especially* when the other people are older and more experienced. Such a person who can really handle that tricky position well -- probably an experienced poly person in your own age bracket -- is likely to have one or more old flames, lovers or partners that they'd be loathe to discard for a couple of new people and an untried triad. I'm not saying it'd be impossible to find such a person... just hard. Maybe a divorcee, or a widow or widower?
You may start thinking now that what if the new person meets someone and wants the chance to explore their feelings with that 4th person...that would not be something we would be willing to be a part of. We aren't looking for another couple, we aren't looking for a quad relationship, or communal marriage. We simply want one more person to be married to. And if that person is married to us, then there would be no others but us. We want a wife, or a husband, a faithful spouse for the rest of our lives.
To be honest, I do find this bit really, really selfish. You two have been involved with another couple for years, so you *know* that commitment and sexual exploration with others are not mutually exclusive. So why demand sexual fidelity from your new third when you've been living happily without it for so long? Why assume they can't be serious with you and still be interested in someone else? Why leave the door open to continue being involved with the other couple you two have your long-standing affair with as long as it's ok with the newbie, but bar the door to anyone else they like?
You two have been together a long time. You have your established patterns and rhythms, you know with absolute certainty the role you play in each other's lives. There's great safety and comfort in that. No matter how welcoming you try to be to your new third, it will take a long time -- years, if ever, before he or she really feels on the same "level" as that with you (after all, I'm sure it didn't all happen overnight for you two!). So under the scenario you've described, you two get the security of your settled love AND the thrilling, scary, exciting, tumultuous experience of new love... while the new person only is allowed the latter. How is that supposed to make him or her feel like they're really being treated fairly? You two got to decide for yourselves that you were ready to open up to a new love without it riskng your existing love... how is it that the new person is never to be allowed to make that same determination?
I'm absolutely sure you guys have pure hearts and only the best intentions, I am. But what you're looking to do will radically alter your existing relationship and build brand new ones within the triad you look to form. That can be wonderful, but it wil also almost certainly be unpredictable, and it will take a great deal of discarding of preconceptions and openness to what comes, and to the fact that it may not look like what you expect. With what you've described, you guys make all the choices, set all the standards and rules, and the new person can take it or leave it. I don't see that as authentically building a new, equal, three-way relationship... I see that as trying to find the simplest, least threatening way to add a new person to your existing relationship like an accessory, like (as you've described above) a "love insurance policy" for when one of you dies. The reason I've taken all the time to type this out is to convey one message -- it's likely gonna be more complicated than that.
Btw, a "vee" is when one person is partnered to two people who aren't partnered to each other.