We get couples like this here every week, sometimes several per week, coming in new to poly, extremely excited about their fantasy/plan of "adding a person to their relationship," or "looking for our third." "Let's create a throuple!"
We often share an article with these people called, "So Someone Called You a Unicorn Hunter?" It is a great article, but long and complicated.
I just found another one that sums things up much more simply:
"Triad or V-- What's More Complicated?"
www.readyforpolyamory.com
It explains, with charts, how a triad is MUCH more complicated then a V, or even an N (with four people).
A triad has seven relationships: four romantic relationships and three metamour relationships.
A V has only three relationships: two romantic and one metamour.
An N has only five relationships: three romantic and two metamour, despite having four people, not three!
We often share an article with these people called, "So Someone Called You a Unicorn Hunter?" It is a great article, but long and complicated.
I just found another one that sums things up much more simply:
"Triad or V-- What's More Complicated?"

Triad or V - What's More Complicated?
I get a lot of people new to polyamory insisting that they must date together, or meet people together, because it "will be less complicated." I see this in comments on social media, on dating app profiles, on couple's profiles on social media (that black and red one that starts with an F...
It explains, with charts, how a triad is MUCH more complicated then a V, or even an N (with four people).
A triad has seven relationships: four romantic relationships and three metamour relationships.
A V has only three relationships: two romantic and one metamour.
An N has only five relationships: three romantic and two metamour, despite having four people, not three!
In a triad, there are 4 romantic relationships. The one between Alex and Bob, Bob and Carol, and Carol and Alex, and the one between all three together. Each of these needs separate nurturing and time to grow, and each dyadic relationship grows at its own pace.
If you're "seeking a third," this implies that one of these relationships (let's say the one between Bob and Carol) has a longer history. So in being partners, they're more stable with one another, but in being each others's metamours now, Bob and Carol have to figure out how not to compare their growing relationships with Alex. They have to do this while still letting the triad connection grow, and not letting all their couple time become processing time about the new relationships.
It's hard not to compare when you're dating the same person. It might seem simple to say, "Okay, we'll spend all our time together, then." But [if you do that] you're making Alex an accessory to your relationship and not actually building individual connections, which isn't fair to her.
There are 7 relationships in any triad: 4 romantic ones and 3 metamour relationships. [The metamour relationships] are usually in the backseat, but end up mattering sometimes, when one new dyad connects more quickly than another and the processing part of these relationships comes to the forefront. People often feel a LOT of jealousy over watching small differences in a relationship of someone they're also engaged with up close and personal, and this can take a lot of big processing talks to work through and find the roots of.
This assumes, of course, that you meet someone who is totally down with "a package deal" and has feelings for both of you that turn out to stay present...