Although it may seem like a completely obvious success now, you have to remember that 'Deadpool' wasn't exactly a sure thing before it was released.
Indeed, Ryan Reynolds had to jump through some hoops to get it made in the way it needed, not to mention sticking with the character long after others had given up. The results are plain to see, and while the sequel may not have had the same sharpness, it still was an enjoyable action-comedy romp.
However, since Disney took over Fox, many were of the opinion that 'Deadpool 3' wasn't going to happen, primarily because the movie wouldn't work quite so well unless it was rated R in the US. Disney have typically tended to avoid movies with more adult themes and content, especially in the Marvel Cinematic Universe, where appealing to as wide an audience as possible is key to their success.
Deadline is now reporting that a third 'Deadpool' movie is now on its way to the screen, with Wendy Molyneux and Lizzie Molyneux-Loeglin on board to write the screenplay. The Molyneux sisters are best known for 'Bob's Burgers', where they've served as writers and executive producers since the show's inception.
Deadline's report also states that 'Deadpool 3' is expected to remain rated R, and that a new director will be lined up as 'Deadpool 2' director David Leitch is booked straight through 2021. The real question is where and how the character will fit into the Marvel Cinematic Universe, if at all. As a character, both on-screen and in comics, Deadpool is aware of himself and frequently breaks the fourth wall to address the viewer.
Whether or not he does or doesn't join the Avengers is irrelevant if we can get some moments like this in the third movie.