After 1. e4 c6 2. d4 d5, what are the differences between 3. Nc3 and 3. Nd2?
It seems 3. Nc3 is more popular, though after 3. ... dxe4, I failed to see the difference.
3.Nc3 allows White to play the Two Knights Attack with
2.Nf3. It does block the c-pawn, though, so Black can play
3...g6, the Gurgenidze System, because White cannot protect the d-pawn with the c-pawn.
There is also the rarely played Gurgenidze Counter Attack
3...b5. A very dubious line that can be stopped easily with
3.Nd2 tries to avoid these lines, but it makes the Two Knights Attack impossible, a line that is very popular nowadays. It also blocks the view of White's dark-squared bishop, so Black can develop their kingside knight to
h6 if they want.