Isn't that self-contradictory? I mean, the more holes a given character has in their toolset, the more they would need to be backed be the absolute best assist/support characters to stand a chance. The further you put them down in the rotation, the worse off they are.
I'm not following you. It's not like any character needs two good assists to function. In general, there's always one assist out there that compliments any two characters. When in doubt, I tend to pick Arthur for Dagger Toss since he's a good anchor and his assist, when powered up, is IMO a top 5 quality assist.
Regarding your statement of any two characters you'd like plus one great assist = functional- I disagree, at least for certain characters. One thing great about morridoom is that it acts as a fantastic gatekeeper team- it's a potent combination and it's becoming more and more widespread, both at weeklies/tournaments and online. For any team I think of, I immediately ask myself: could this team stand a chance against that sort of keep away? If the answer is no, I don't consider the team functional. So the ANY two characters plus a good assist formula rings false to me because if the two characters you like are hsien-ko and she-hulk, there's no character in the game that good possibly offer enough support to make that duo work.
With Hsien-ko and She-Hulk, you already have a good assist! Plus Hsien-ko is excellent for setting up DHC baits to She-Hulk. No one gets in on Hsien-ko without superjumping when you back her with a good assist. I'll never say that Hsien-ko is a good character, but I've OCVed plenty of people with her.
I also find it ironic that you chose Hsien-ko because she's one of the few characters who can completely shut Morrigan down. Morrigan simply has no answer to the gongs. If you give Hsien-ko a good assist (personally, I like Hidden Missiles a lot), she'll have complete control of the entire match. Hsien-ko is also one of the characters with the most air delay time, letting her run out the timer on Astral Vision if she doesn't feel like dealing with it.
For Hsien-ko/She-Hulk, a strong horizontal assist clearly takes precedence to hulk She-Hulk get in. Hsien-ko would be okay with just a vertical assist. Hidden Missiles is not a good option because She-Hulk can't protect it.
I would likely either choose Hsien-ko/She-Hulk/Arthur or Hsien-ko/She-Hulk/Dr. Strange. If Shuma-Gorath weren't such an awful anchor, he'd be up there too. Doom with Plasma Beam is not a bad option either, since he makes good use of She-Hulk's unblockable and can call armored Senpu Bu at any time during his j.f+H to give him absurd pressure. Arthur also has really fantastic DHCs with Hsien-ko.
The irony if your She-Hulk/Hsien-ko choice is that you picked two characters that are actually very good against a variety of matchups. Hsien-ko is
the anti-zoning character of this game (though Dormammu destroys her), and She-Hulk is a good anti-rushdown character, especially when backed by armored Senpu Bu. In your first match, put She-Hulk on point because you don't know the matchup and use her high health to guarantee a DHC into armored Hsien-ko if things aren't going well. IMO, once you have armored Hsien-ko smart play should win the game for you. LLND very rarely loses if he has Rimoukon up, he usually only loses when Hsien-ko dies before she goes armored.
Other neat options are Chris and Taskmaster, since they have good horizontal assists and back Hsien-ko up well. Taskmaster is an especially good choice because he makes good use of Senpu Bu with Shield Skills -> Aim Master L and She-Hulk's low assist via Spidey Swing -> j.M.
I'm not seeing these two as a terrible combination.
edit:
@Dahbomb- I don't think a team can be designed to cover EVERY possible match-up, but it needs to deal with the common stuff fairly well, IMHO. In vanilla, there was no point to designing a team if it lost to wolverine and/or phoenix. In ultimate, morridoom has taken their place (though I don't think morridoom is too good, like vanilla wolvie and phoenix clearly were).
I think there are teams that cover every possible matchup. Personally, I feel as though my previous Vergil/Dormammu/Morrigan covered every matchup. Morrigan + Vergil is fantastic for anti-rushdown, Dormammu+Morrigan covers aerial characters, and Vergil+Morrigan covers keepaway teams by letting you Spiral Swords through anything. I never, ever felt outmatched with that team.
We need a 'two and glue challenge' or something. Like somebody has to pick two characters who they think are trash, somebody else picks the third character and designs the team for them, and then they have to go online and prove it stacks up.
It's your claim that it doesn't work, so I say you should take up the challenge to come up with two characters that won't work with a third assist. ;-) I guarantee you that in almost all cases, Arthur will make that team solid.
That would require you to know how to play those characters first. I guess Karst and Frantic can take up the challenge since they have some wide character experience....
Better yet best man for the job is GB.
I don't have the time to learn a new team though.

I'm busy figuring out my own!
Two and Glue huh? Phoenix Wright and Jill will be my picks. Yeah I know about Jill "secret tech" talk, but she's a lot of work to get that damage so I don't think highly enough of her to care.
Any Ideas for a third for Phoenix/ Jill
I would undoubtedly go for Plasma Beam. People underrate Wright's assist. He's the only character in the whole game with a full screen projectile that
also hits low. Put him with a character that has a lot of overheads and it's very frustrating for your opponent. I think MODOK with Balloon Bomb assist is also a good choice. It's a great footsies assist for Jill, and just sits there to protect Wright. Shield Barrier would be an interesting choice, too. Sentinel Force could work if you're willing to burn meter with Jill to get in (which you should be).