Sadly I’ve been doing a little internet sleuthing and come to the conclusion that Brolin won’t be in Deadpool 3. Or, if he is, it’ll just be a small cameo. Josh Brolin is currently filming Dune 2 in Budapest which will run into DP3’s production schedule, as they start filming the latter in January. From there he’s jumping right into the second season of his streamer, Outer Range, so it’s doubtful he’s going to be appearing in DP3 as it seems like his schedule is very busy. I suppose they could always recast, but if they bring back Zazie Beetz (along with Hugh Jackman who’s already coming back as Logan) I suppose it would be a bit weird to recast just Cable.
My guess is we’re out of luck regarding a Brolin Cable return in this one. At least in any major role that is, but hopefully they can slip the filming of a cameo into his busy upcoming schedule. Fingers crossed, but if they’re going for any X-Force squad in this film it’s probably a combo of the Kyle/Yost and Remender Uncanny X-Force versions… so Logan, Deadpool, Domino and maybe X-23, Vanisher (as there was a rumor going around that Brad Pitt has been approached for this film), etc.