Thank you Dino!
"i would use array list and loop thru it every frame."
Isn't this expensive when you have tens of objects on the screen?
Additionally, you occur into the risk that the objects get deleted wrongly, it needs a lot of check in my opinion.
Anyway, I solved in a way I think it's quite classy: added a trigger to the objects, and when one of them touches 2... well it means that 3 are in contact, so I proceed with the due consequences!