I figure that if Marco breaks up with Jackie and Janna is enough of a slut to try and jump on Marco. Then Tom will be an equal scumbag and try to make his move on Star. Then you have two fronts dealing with the same problem. But that's definitely not the tone the show is going for and I have a very hard time imagining that we would in fact see something like that happen.
Much more likely is just that Jarco won't break up all the way till they actually stand to fight Ludo/Toffee and that's why she wasn't present in the opening.
Something might happen that would cause Jackie to back down and go back to Earth before the final few encounters.
Once the final act is done, the crew return victorious and Jackie tells Marco she's not dealing with his cross dimensional bullshit and Star saving antics so we get a set up for Starco as S3 ends.