Okay, yes that's definitely just an estimate. And it is possible that the connection could go a little beyond 3rd cousins, but not far, and it's not very likely. Since you've checked through your g-g-grandparents, then Dennis's comment seems relevant. A good way to identify an NPE (used to mean "not the parent expected", now means "non-parental event") is to look at residences of all of the people in your tree and find two people who were very close together at the same time. Otherwise, you can mine your DNA -- look at the people who match both of you on the same sections of DNA where you match each other. If you can find such people and they have enough information posted to build out their trees, you might find a common ancestor to yourself or to your match. That would be the golden ticket to prove which one of you has an NPE in the tree.
It is best if you can get precise data about how you and this cousin match -- which segments of which chromosomes, and how long they are. I don't know how the Geni/MyHeritage partnership works. It may be that your kits are in the MyHeritage system already. If not, you can download raw DNA data from all places I know that do tests, so probably there is a way to get your data from Geni. You could then upload to MyHeritage and/or GEDmatch and/or FTDNA. If both of you have kits on any one of those sites, you can then get precise data about how you two match each other. If you can give us the total cM you share, it could help eliminate the possibility that your match is beyond the third cousin level.