I'm not sure if I understand you. B3 games are played in arcade mode (no TIE, no CT), minutes are not counted for training. So teams can set the best lineup they have for B3. So there is no impact on league and cup games.
there is an small impact, if you take it serious - you could count with an TIE at least in the tuesday match.
Another question, when you make this cup to make a promotion more attractive, isn't there an risk that a demotion gets to attractive too? Which will at least make the 6 and 7 place more attractive, because you get the chanche on a international cup if you fail to stay in league.