Deadpool 3 is already in works.

Since a great success of Deadpool, it is not surprising that Fox decided to make a sequel, thanks to Mr. Reynolds. The movie suffered a loss of director already. Tim Miller dropped of the project over creative differences. And of cause Reynolds who literally gave birth for the most successful X-men movie ever made would dictate every step his movie will make. Even adjusted it beats every movie Fox made yet in that franchise. 

Of cause if Ryan Reynolds was not persistent enough this movie would never happen, because the studio didn't want to "waste money for doubtful project. And now the should apologize for being faithless in original projects.

And now, when main X-men trilogy is over, it might be that Deadpool is the only property left that will have sequels soon. So Fox in a bigger debt with Ryan Reynolds for that movie than in previous circumstances.

As for now, they know what they are doing with this franchise very well. Second movie will come out in March 2018, so there is clear that for them making the third movie at some point is a giant understanding of where the franchise will go. Using that they will have background to start more risky projects like Gambit or maybe even rebooting the main X-men franchise again.