Leaking heater core?

Either they are defective parts (are they OE or aftermarket?) Or they are not being installed correctly. I would look at the ones that leaked and see if they have a common failure point, like something rubbed on that spot or a seam that failed the same way. Those are about the only reasons why you get repeat failures.

Replace the core with a factory core after inspecting the other cores to determine why they are failing. Stop leak will plug the core up and can cause other issues. The only time I see numerous failures on the same part is usually if they are defective parts, especially the ones from the auction site.

It is inside the HVAC housing that is behind the dash. To get to it you recover the AC charge out of that system, drain the coolant (or clamp the two lines to the heater core) then you unbolt the dash panel assembly and remove it, the floor console if it has one and then you can remove the HVAC case. Once it is out you remove a small panel and remove the core. Then you reverse the process to install the replacement. I would think the dealer would be using OEM parts for these repairs, so that would mean either there is a defective batch of them, or they are not being installed properly. With the rapid failures they should all be covered under warranty at least. You might want to talk to them about the failures and see what they say as this isn't a normal failure pattern.
These are a couple of videos of the process that show much of what is involved in getting the case out.
https://www.youtube.com/watch?v=ZlevrFYwBj0

https://www.facebook.com/akautomotivepeabody/videos/528443514733601/