Marvel Ultimate Alliance 3 has two difficulty modes that you can choose from when you first start the game. However, if you play through the Story mode and complete the game on either black or blank modes you will unlock Superior Mode.
In Superior Mode you can start the game at a brand new difficulty level. Each stage will be tougher than you found in either the Mighty or Casual difficulty, but your rewards will be better as a trade-off. You will also find that in Superior Difficulty there will be new Infinity Rifts.
You will need to start a new story campaign if you want to play on Superior Difficulty. You cannot just replay select chapters on the Superior Difficulty setting, so you’ll need to start over from the Kree Ship in Chapter 1.
Heading into Superior Difficulty is somewhat like a New Game +. You will have all of the things that you had previously unlocked, including the levels you’ve attained for your characters, the abilities that you’ve unlocked, Alliance Enhancements, ISO-8 Fragments, and everything else that you’ve completed in the game. If you’re looking to further level your characters, doing so in Superior Difficulty is the way to continue the Marvel Ultimate Alliance 3 experience.