Three at the back is shite was the reason today ....but there are many different ones.
3 at the back isn't really the issue.
It's the players and their roles in it.
At least one of the wide CBs has to be comfortable as a full back or a midfielder as there is always space to move forward into. You can't just have 3 players in a line within the width of the penalty box.
You certainly can't do that if your RWB is a forward stood 30 yards further up than you'd naturally expect.
Nor then have a CM move over filling the space where the RCB should be moving into.
It was far too complicated for the personnel.