There could be another kind of cover in the mix for random draws. “Rainbow” covers, where they train all three (or two) of the character’s skills. Make them about as rare as the next tier of difficulty: rainbow 2* would be as rare as a 3*, for example. Maybe give them a higher selling cost, too, so they’re an actually valuable bonus for those times you get one but don’t want to train it.
For extra fun, they could be called “holographic” or “foil” covers, evoking the worst of '90s variant cover excesses.
Cool idea, problem I see arising from this is what if you have your character at 3,3,3 already and want to take them to 5,5,3? A “foil” cover would take them to 4,4,4. I guess there would just have to be the option to not have to use all 3 of the cover powers?
Still a very interesting idea!!