It's not 3 doing the job of 2. The expectations of a back 3 are different to that of a 2. The set up is a 3-4-3, so by your logic we can say that the back 3 is doing the job of 4 and the front 3 are doing the job of 2 (also wrong but just to make a point).He's shouldn't be assessed as a conventional CB in a pair. It's 3 doing the job of two.
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Diers distribution isn't as bad as some are making out, he's not great, he's not bad, he's average. The only reason it's an issue now is because both the other CBs aren't good passers, the RWB isn't good on the ball either and Lloris has woeful distribution. Added to that PEH isn't great at getting into positions to receive the ball from the defence and options are reduced.
The one thing that Dier gives the defence is leadership. None of the others talk much. Romero is obviously our best defender but he can barely lead himself nevermind anyone else.
Davies is also quiet and does nothing in that regard. Despite it being said by some that he drops a clanger each game, he doesn't. Since the start of last year he has made very few mistakes and has been really dependable.