As much as it sucks, selling your 5*s is the wiser option. Yeah they’re hard to come by, yeah they’re the strongest characters in the game, but having them bumps up your scaling by a huge margin and is probably making the back-half of PvE nodes way harder than they’ve been. Plus, they’re never required characters. Definitely want to hold back on opening legend tokens for the time being.
As for your other characters; roster space is the biggest hurdle for new players these days. The burden gets lighter once you’ve got enough champed characters to see HP coming in more regularly, but the first few months are a challenge. If you can drop a few bucks to get the HP needed to roster them. Though if you want to keep it F2P then cuts will be made. Don’t worry too much about the lower-end 3s, once the ball gets rolling you’ll get 3 covers at a decent enough clip that they’ll fill in fast.
If you have to pick from the ones listed Iron Fist should be top of your list. He’s easily in the top 3 for 3* characters and a strong contender for the #1 slot. Scarlet Witch and Khamala Khan are also top tier. Those 3 should be the priority above all the others.
Rocket & Groot, Daken, and Black Panther are also really good choices. They’re all solid and will be workhorses for your time at 3*. R&G and Daken especially for grinding PvE since they have true heals, and Panther has the strongest AoEs among 3s. But unlike KK, Switch, and Fist they don’t see a lot of play in 4s, so less of a long-term investment.
Punisher and Hulk are okay, but their glory days were a long time ago.
Sentry and Rags you’re just going to want to sell for now.
Peggy is really strong and won’t screw your scaling, but it’s going to be a while before she’s at a usable point. Venom I personally like but can’t recommend him ahead of some of the others. His yellow packs a serious punch, but his powers don’t mesh that well together and he’s ultimately only shines against high-level PvE Hawkeyes and sentry drones.
Just sell the 2* Hawkeye. You’re good for 2s, and if you’re going to make room for another 2 then you’ll want Storm, not Hawkeye. And if you’re looking to cut people already on your roster then the 2* Captain America and Squirrel girl are your best bets. You’re pretty set for 2*s and Cap is underwhelming at that level. And while keeping Squirrels is a good idea in the long term, trading her for Fist, Switch, or KK is worth it.