I mean: rosters would be frozen only for B3. If a team bought another player he could be used on the league or cup, but not on B3. Like buying a player during league playoffs (you can keep him but can't play him on playoffs).
You either didn't get my point (people don't need to have B3 finals teams at the start of the season except maybe for B3) or you suggest there should be two different rosters, one "real" for league, cup, sc (and PL) and one for B3 and you would pay for that frozen B3 squad only for the real one. Which surely won'tbe implemented for many reasons.
You have to bne able to sell players wheneever you want. I'm not sure what you are suggesting, let jsut say TEAM X from middle size country has monster A on its roster at the start of B3. That team doesn't need him for league games, it can win without him. So
a) Monster A is frozen on roster and team has to pay from him till he is knoicked off B3
b) Monster A is frozen on roster for B3, paid and can be sold meaning he is no longer on that B3 roster (except when re-aqcuired) nor league roster
c) Monster A is frozen on roster for B3, not paid, unless he is still part of that team league roster and when sold he just ceases to be on league roster and is still on B3 roster
d) Monster A is frozen on roster for B3 and acts normally like on league roster till sold, then he is not eligible for B3 nor league roster anymore that season
None of these options is viable. The problem with frozen roster is overpowered teams at the beginning of the season not underpowered. Since domestic competition is not the same for everyone, frozen teams are not fair enough for everyone.