It exists to show you familiar places, from Hell's Kitchen to The Raft to Wakanda, which in turn exist to stage biffo with familiar heroes, from Ghost Rider to Doctor Strange to Captain Marvel.
This steady stream of new sights is just enough of a hook to tolerate the mundane level design, simplistic combat, and a camera system that ends up being more of a threat than Thanos himself. Just as proceedings start to get repetitive, Black Panther or Wolverine leap out of nowhere. If you enjoy saying "I recognise that!", then this is your game.
Given that, it's a miscalculation that characters level-up individually only while in use. By the end I had no choice but to stick with the same four heroes. As much as I wanted to give my benched level-12 Spider-Man a swing, my level-40 Thor was the only viable choice. This strange design choice ends up working against the game's best feature: the sprawling, iconic roster.
More middling than marvel, but in the right mood, and in small doses, Ultimate Alliance 3 satisfied the base part of me that just wanted to see Thanos get punched in a video game. At the very least it's better than Age of Ultron.