As far as I know there is a lot of drama going on because of Ter Stegen and what he was told a while ago and how the club leadership now seems to have taken a U turn. I can understand his position, he is the captain and thought that wants he has fully recovered from his injury and brings his best performance, there shouldn't be any doubt about his position in the team, but things seem to have changed for some reason.

I don't quite understand it because apart from that injury, Ter Stegen has been a reliable player for so many years. What is the reason that they are now planning to replace him? Or is it just to keep up the pressure for the goalkeepers because every player needs competition on their position to continuously feel the urge to show their best game. That makes sense of course, but they could still give him a high trust level and communicate it to him. Misunderstandings aren't beneficial to anyone.

Since Ter Stegen has no plans of leaving and Szczesny is still hoping on fighting for a spot, i think they're still the first and second choice but on the arrival of Garcia either of the have to be dropped for the new goal keeper to be giving playing time cause there's no way Hansi Flick would buy Garcia just to keep him on the bench, those other two have had their moments maybe this is Garcia's time.

 Well one thing am sure of is that regardless of who's the first choice for the new season their would be rotation among them but one of them would be on the bench mostly as third choice and I guess it would be Szczesny since he's the oldest amongst all.

This is difficult to implement because goalkeepers with more playing time typically have a stronger mentality and confidence, so Hansi Flick can't afford to do this, as it would pose significant risks.
Having three high-quality goalkeepers is certainly difficult, but Flick can still rotate and compete among them for starting spots. This is also likely a measure taken to prevent injuries and allow other quality goalkeepers to cover for them.