I haven't played UMvC3 much, but I have watched a ton of videos and am well-versed with fighting games in general. Thus, I've decided to come up with a tier list of my own.
Above everything else, keep in mind that tier lists should not be focused on at any level of play. This is just to categorize each character in terms of in-game capabilities and in no way takes strategy into mind (with one exception, see below).
Top Tier+: Zero, Morrigan
Top Tier: Wesker, Vergil, Magneto, Spencer, Dr. Doom, C. Viper, Wolverine
High Tier: Dormamu, Amaterasu, Frank West, Dante, Felecia, Akuma
High-Mid Tier: Taskmaster, Strider Hiryu, Viewtiful Joe, Super Skrull, M.O.D.O.K., Hulk
Mid Tier+: Jill, Firebrand, Spiderman
Mid Tier: Hawkeye, Trish, Haggar, Chris, X-23, Nova, Ryu, Storm
Mid Tier-: Thor, Nemesis, Captain America, Deadpool, Rocket Raccoon
Low-Mid Tier: Dr. Strange, She Hulk, Chun Lee, Sir Arthur
Low Tier: Tron Bonne, Iron Man, Shuma Gorath, Iron Fist
Bottom Tier: Ghost Rider, Hsien-Ko
Additional Tiers that don't fit into the standard scale
Cheap Tier: Phoenix, Sentinel
Wildcard Tier: Phoenix Wright
Q: HEY! WHY IS <insert character here> <insert tier name here> TIER? DO YOU LOVE/HATE THAT CHARACTER MUCH?
A: This list is based off of my observations including total damage output, priority, cross-up potential, ease of use, and potential combinations with other characters, as well as tournament performance. If you play Hsien-Ko, then that's fine even though she's bottom tier. Tier lists for fighting games only show how well characters match up on paper.
Q: What's with the additional tiers?
A: Phoenix and Sentinel are actually High Tier and Mid Tier respectively, but they have a highly exploitable attack or ability that must be dealt with in a specific way. Phoenix Wright is an extremely unpredictable character due to how his stances work and thus cannot be firmly categorized in any way, similar to Roll in Marvel vs Capcom 2.
Q: Why is Morrigan Top Tier+?
A: Normally I would count this as question 1, but Morrigan is a bit of an odd-ball. Mainly, it's due to how she combos with Dr. Doom to essentially turn UMvC3 into a bullet-hell game. Although this would technically qualify her for Cheap Tier, her bullethell strategy is difficult to pull off and thus not easily exploited. Without it, she would be mid tier at best.