Generally the 3-4-3 is a good formation and it suits various German players. But we still lack the right material on some key positions. Very questionable to not call up Baku when he intended to play this way. This would've freed up Kimmich for the CM and we need somebody with energy besides Kroos if we play this formation. It would've allowed Löw to just mirror Chelsea with Jorginho (Kroos), Kante (Kimmich), Chilwell (Gosens), James (Baku, Mount (Gündogan), Havertz and Werner. Anyway, he has to play Müller now and he doesn't fit into the 3-4-3 either and keeps Werner out of the team.
IMO he should switch to a 4-3-3 now and just do what Pep did with Cancelo. Play Kroos in the Rodri role, let Kimmich take over a CM position in possession and Gündogan push higher. His biggest strength this season was his goal threat when he attacked the box and he said himself how important it was for him that Cancelo and Zinchenko took over his spot to give him more freedom in attack. Of course you need an attacking player to hold the width, then, so maybe bring in Sané (although he's playing terribly) or use Werner wide. Not optimal but I think it would improve us generally. You could even play a 4-2-3-1 and bring Müller in his favourite position behind the striker.
Maybe:
-Kimmich-Hummels-Rüdiger-Gosens/Can-
-Kroos-Gündogan
-Gnabry-Müller-Sané
-Werner/Havertz-
Or
-Kimmich-Hummels-Rüdiger-Gosens/Can-
-Kroos-
-Gündogan-Müller/Goretzka-
-Gnabry-Havertz/Werner-Sané-
And as I said, important to consider that the fullbacks play "invertedly".