Does anyone know if and incase who among the 3*s that give coulson as champion reward?
Tricky. We already have Black Widow to Nick Fury and Hawkguy to Kate Bishop (to go by MCU SHIELD associates). Quicksilver seems like the best choice to me, because tile swapping. Or, since there are more Legendary than Rare, maybe there won’t be a direct link. Not for a while anyway.
No 3* currently gives Coulson covers as a reward. I think just Colossus and Patch (X-Force Wolverine) and Daken and Ragnarok (X-23) are the only 3* characters who are doubled on rewards still. Oh, and Gamora and 3* Star-Lord, but I’m sure it will be revised that Gamora will give G4mora at some point.
The 4* tier now outnumbers the 3* tier, with no indication that will change anytime soon. Naturally, some 4* characters will be left out as champ rewards from 3*s.
Hmm, maybe they could change Doc Oct to Sandman? I mean, unrelated, but a more appropriate popularity link as far as character power is concerned, neither being considered all that good, and both classic S6. The problem being, where would Carnage go?
Thats to bad 
I would think Ragnarok giving Sandman would be a better fit. Villain to villain is about the only connection, but as I already mentioned, Ragnarok’s one of the few characters still giving the same reward as someone else, and it would involve less shuffling/coding than if you changed Doc Ock to Sandman then changed Ragnarok to Carnage.
They’ll probably change it eventually. Their current gameplan seems to be to release a bunch of characters, then at points swap 3* rewards around to reduce dupicates and add the new people in. They also tend to move things to make more logical jumps.
However they have also grown the 4* teir beyond the 3* tier so there literally arent enough characters at the 3* level to give out every 4*. Currently there are 52 4* (50 if you remove the 2 limited) and only 44 3s. Despite this there are currently 3 4 avaialbe from from multiple 3s still (St4r-Lord from 3 Star-Lord * Gamor4, XfW from Colossus and Patch, and X-23 from Daken and Rag) so there’s room for them to add 3 more 4*s, cross your fingers and hope I guess.
Let’s hope maybe the stars will align and they change the champ reward of 3* so that each 3* can give more than 1 4* character since now there are more 4* and 3* 
While that would certainly be a welcome development, I would never, ever bank on them being that generous.