No idea if this is worth anything, but I’m up to 7 maxed 3*s. This is the order I went
LCap (first one I got maxed covers for)
Patch
Daken
LThor
Sentry
Punisher
Hood
I bought 4 covers for Daken, 2 for Sentry. Everyone else was just as I won covers (and the ones I bought I have since won). Currently I’m sitting on the following with full covers (but not all covered ideally):
Falcon (needs 1 yellow to fix), Psylocke, GSBW, Torch, Hulk, Shulkie, Spidey, Doom, Rags, Loki. BP needs one more of any color (but 1 yellow, 1 black to be correct).
Right now I’m just hoarding ISO since none of those feel like they’ll make my gameplay significantly better.
If you’re not going to buy any additional covers, the most useful one might be Patch. He’s missing a lot of yellow, but he still has fast regen out of matches, , he’ll regen a tiny bit during and he’ll sit under Daken on green until he’s maxed so he shouldn’t tank too much. Unfortunately, that means his red won’t be as powerful for you as it could be.
GSBW can be a powerful character, but she sort of needs 5 green to be as effective as possible, and even then, 19 is a lot of green. And she’s fragile. Torch is nice, but also fragile, but his red is pretty solid, and the black is no slouch either.
Once you have patch/daken maxed you can play a fairly long time without heavy reliance on health packs, which can be nice in PVE. They’re not the most scary on defense in pvp, but in pvp, any team can beat any other team if they want to.