London System with Nc3 -- what's the idea? Recently top GMs are playing a London system type setup with Nc3 before c4.  I can't imagine they are going for cheap tricks with Nb5, but I am not seeing a valid positional idea behind the move either.
For example Carlsen-Ding 2017 

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1.d4 Nf6 2.Bf4 g6 3.Nc3 d5
If you want to know whether the knight is badly placed on c3, you have to think about your plan. Normally, in d-pawn openings you want to play c4 to attack Black's hold on the center on d5. Here, this is not White's plan. Instead:


White may want to play h4-h5 and attack Black on the kingside. This means White may want to follow up with Qd2 and 0-0-0.
Another idea is the play a gambit with e4. If Black takes dxe4, the knight isn't so bad on c3.


The badly placed knight on c3 is a small price to pay for "getting it out of the way", since it wouldn't do much for the attack elsewhere (it's block in the queen's best square on d2, and a3 is an even worse square).

You are right that according to conventional chess wisdom, you shouldn't play like this. But this kind of system is not conventional! Highly creative players like Jobava play this way (In fact, this setup is named after Jobava). Check out some of his games here.