These are the necessary resource cards that each deck should include a single copy of in all of their decks at the moment; Energy, Genius and Strength.
The setup turns in the game are very important. If you see these resource cards in your hand at the beginning of the game, then you should not Mulligan them away. There are very few cards that will give you additional resources from a single card. This will give you a better chance to get multiple things out on turns when you see them. And the first turn can often be a setup turn to get out an ally or something like a Helicarrier/Avengers Mansion. Perhaps you get out a second card as well as one of those cards because you have the extra resources to do so.
Some heroes will rely upon specific types of resource //. Even if a hero only cares about one type it still makes sense at the moment to include all of them because the resource advantage is so needed right now.