I suspect the change to a back 3 is in large part because Switzerland themselves play a back three, and we had difficulties pressing against Serbia and Denmark's back threes with the shape we were playing.
I think the 3-4-3 - or 3-4-2-1, whatever you want to call it - will force us to make some awkward selections. We don't really have two wing backs. I expect it'll be Trippier and Saka rather than TAA; I'm not sure Trent as a wing back really works. I think there's also a chance that Gallagher will come back in alongside Rice because Mainoo maybe isn't good enough out of possession to play in the midfield 2, and Switzerland are pretty dangerous in possession.