A longer trilogy wouldn't have helped since the first two films were also going to be the Dark Phoenix story, it's just been compressed into one movie.
So we'd still have the franchise hurtling into Dark Phoenix. It would be more comic-accurate with Hellfire Club and Shi'ar but we've no real idea what Kinberg had in in mind.
The problem is that Xavier, Magneto and Mystique were given fairly complete arcs in the first three films (First Class, DoFP, Apocalypse) so that should have been the end of them having to take up masses of screen time.
Xavier lost use of his legs, lost his hair and encouraged Jean to embrace her powers this time; Magneto went to the dark side but ended up friends with Charles once again; and Mystique didn't end up being an evil villain and instead ended up joining the X-Men. Having her leading the team was unpalatable and didn't fit the character at all, but nonetheless she ended up at the mansion. They had their story arcs.
In Dark Phoenix, they should instead have focused on the X-Men themselves, something they have failed to do over and over again...