Power house three stars

While all of the three stars are "good", here's the ones that might be a bit overpowered that you should keep an eye out for. These aren't in any particular order.

Captain America: With his blue ability to leave gigantic protect tiles and a 5/5/3 setup (3 in yellow) that lets him replace any tile with a countdown, he's perfect for the PvE events. He also has a really good HP pool.

Daken: He's ridiculous in terms of difficulty to kill. You definitely want to get his black and purple abilities up so that he can make tons of strike tokens and heal. His green and black attack damage are usually enough to tank with those abilities which can make him a very good damage sponge.

Spiderman: By going 3/5/5, he can make any purple match into a giant shield while also nearly chain stunning opponents. Doesn't deal much damage, however.

The Hood: 5/5/3 is what you want with him. Dormammu's aid will keep opponents deprived of resources which is an amazing ability. Also, his twin pistols, while expensive, can be used to generate a ton of tiles during the fight (you do gain the AP when he uses it.

Magneto: 5/5/3 but almost certain to get nerfed. As it stands right no, as soon as you get 5 blues you basically win the game. He can match 5 blues to get 6 back and his red lets him change the board state when he finally gets stuck. It's not a hard lock but, nevertheless, it's a VERY strong combo. Note, while he uses pink abilities he doesn't actually do pink damage (he's really red/blue/yellow).

Black Panther: Not super strong but if you're transitioning from 2 star world, he was very useful at that time for me because he provides a use for black/yellow tiles and can fit in well with OBW and Wolverine (2 star).

Patch: The 3 star wolverine variant is ridiculous. He does a ton of damage and auto-heals. Collect him.

Falcon: His yellow ability can be RIDICULOUS with people who put out a lot of special tiles (like say a Daken / Spiderman team?). He also takes out enemy countdown tiles with his blue so he's another good PvE grind candidate.

Thor: Even more annoying then his 2 star brother.