Triangulation is the process of proof, Emma. What is important is that you find both, ancestral segments through triangulation and your can connect them in your family tree roughly the estimated shared DNA percentage (though that varies for distant cousins due to the randomness of DNA recombination and endogamy).
The more DNA cousins can be found for the triangulated group the better (in general). But usually we don't have a link to the MRCA (most recent common ancestor) or CA (common ancestor) and due to limits in the family trees (our own or those from our DNA cousins) this turns out to be the hardest part.
The important things for triangulation is 3 independent (meaning not recently related) individuals having minimum one an overlapping (ancestral) segment.
That's proof enough that you are related.
The only challenge is to find out how you are related and that's where the family trees come into play. There are several factors that can make it complicated, endogamy is one of them (can be that your parents are related to each other or one or more of your close ancestors). Eventually everyone will find a case of endogamy, based on the principle of pedigree collapse ("Ahnenschwund").
But as we keep on adding more and more DNA cousins to our triangulated groups, including close family members to determine on which branch of our family tree the TG exists, it becomes more and more clear which region (geographically) and eventually which couple is a MRCA or even a CA. Even if we don't have a tree it makes sense to work with our DNA cousins as others might have overlapping family trees early on and we can then research together with our DNA cousins to enlarge the family tree (including side lines) and thus connect more and more of our DNA cousins to it.
That's why it's also important IMO to add matches that triangulate and are small (between us and our match). Because that doesn't mean that amongst our DNA cousins they all have small segment matches as well. Very often I found that at least two of my DNA cousins in a TG have a large match (meaning 1st or 2nd cousin) which allows us all of a sudden to advance in our TG as we can identify the MRCA that way.
Good luck everyone!