They can get away with casting Hugh Jackman in the film as either as himself or in some non-descript cameo ala Stan Lee. Like if Deadpool gets a package delivered to him, the delivery man could be Jackman. Or he and Cable eat some food via Uber Eats and the delivery guy is played by Hugh. Another scenario is that he and Domino and Cable could have drinks at a bar and the bartender is played by Jackman. Or if they want him in a significant role, he could play a documentary film maker who is fan of Deadpool and wants to document his every move so Deadpool doing "4th wall breaks" would actually be him talking to the documentary film maker.
So if he's Deadpool 3, they can cast Hugh Jackman that way without casting him as Wolverine. People think that if he's going to be in Deadpool 3, they automatically think he's going back to play Wolverine, when it could be that he plays a non-descript character in a cameo.