Deadpool And Wolverine Will Cable Be In Deadpool 3

To answer your question right away, Cable does not appear in Deadpool 3.
If you have watched the movies that came before Deadpool and Wolverine, by the end of Deadpool 2, Cable uses the last of his time machine fuel to go back home to save Deadpool from dying. Since we do not have a confirmation of him making it back to his time, many fans assume that he is still stranded in Deadpool’s timeline. This is the main reason why there were talks about his appearance in Deadpool 3.
However, even though Josh Brolin did not appear in Deadpool 3 as Cable, he might still make an appearance as the character in some future Marvel movies since according to Kevin Feige, movies that will be released after Deadpool 3 will feature more involvement of the mutants. So, let’s wait and see if this happens or not.