The ending

After the second movie, Poiré and Clavier (respectively director and main actor and both wrote the screenplay) preferred to work on the American remake as they, along with French studio Gaumont, had great hopes of making it big in the US.

"Just Visiting" turned out to be a nightmare for every French involved. Originally, the movie had been greenlighted as a coproduction with Disney through John Hughes (Home Alone, Flubber). Then, Hughes backed out of the deal and studio Gaumont couldn't cancel the project. The company was then obliged to finance the entire movie alone while they couldn't get a real distribution deal in the US. Of course, it flopped and it even left them close to bankruptcy.

Director Jean-Marie Poiré took an Alan Smithee and was credited as Jean-Marie Gaubert (he's still Poiré in the screenwriting credits). He apparently swore to never work again with Gaumont, even if he had a very long story with them (his father, Alain Poiré, produced most of their mainstream movies for more than four decades). His next (and currently last) project, the awful "My Wife Maurice" (trust me on this one) was produced by Warner Bros and his new one should be made for another studio. Gaumont own the rights for the story and characters and few people would be interested by a movie that's either a third part while the previous episode was a mild success ten years ago or a sequel to a big flop.

So,I'd say it would be easier for the Beatles to reform than for a "Les Visiteurs 3" to be made.

Yes, Les Visiteurs 3 is coming.