God if the Enclave did make a return with their army to fight the NCR and Legion..... well that would be a never-ending war. And the NCR and Legion may just have to team up to stop them.
There's barely any Enclave left. Their major bases of operation (Raven Rock, Adams Air Force Base, Navarro, The Oil Rig) are all gone. And while I would expect them to probably have lots of bunkers, and definitely they are at NORAD. But their army was never big, they relied on superior tech. If there is any Enclave left, they will either be like the Brotherhood in this game and stay locked up in a bunker (NORAD), or groups like the Remnants. The major players in the Fallout Universe are definitely NCR, the Legion, and the Brotherhood (kinda)
As for the topic, I united all three to fight a common enemy. However I wouldn't actually consider the Remnants to be "true" Enclave. I'm guessing those five veterans will either retire or work with the NCR after the battle.