If one team win both Cup and 1st Division League, the runners-up of both tournaments could play a game between them (probably on monday next to the league's 3th game) to qualify one of them to the B3
I know the "what if someone wins the double" scenario is being discussed, and I actually brought this idea up, but the BBs would prefer to not schedule extra games.
It's a real simple solution: if one team wins both competition, the next eligible team in the 'lesser' competition is chosen. In this particular case, this is the finalist in the national tournament. No need to schedule or play extra games. Either way, it would only be fair to have the two teams qualify one from each competition.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."