scvchess wrote on 02/04/21 at 13:45:02:
First an answer to one of your specific questions. You don't need to worry about the move order 1.Nf3 d5 2.c4 c6 3.Nc3 because you have 3...d5. In general I would be careful about playing ...d5 because you can get reversed Benoni positions, but here since you get tempo on the knight it's just better for Black. The more accurate move order would be 1.Nf3 d5 2.c4 c6 3.e3 Nf3 4.Nc3 at which point you would be best advised to play 4...a6 allowing a transposition to the Chebanenko Slav. These systems for White make it hard to transpose to the Classical Slav, it's either Semi-Slav or Chebanenko.
Generally, it's fine to add information to a thread six years after the fact, but conversationally addressing the OP sounds a little odd since they have almost certainly moved on to other questions after all this time.
Specifically, indeed 1.Nf3 d5 2.c4 c6 3.Nc3 can be answered by 3...d4, but the OP might have just been typing hastily, similar to what you did with your typo "3...d5". White can instead just play 3.e3 Nf6 4.Nc3 as the Anti-Slav players do these days, and in fact this transposes to your second "more accurate" move order. The ones advocating 1.Nf3 d5 2.c4 d4 were trying to suggest a good line for black that also avoided the Semi-Slav, which your 1.Nf3 d5 2.c4 c6 does not appear to do.
The Semi-Slav is a good opening and as you noted is pretty much immune to move-order tricks. Even a Slav specialist like Jonny Hector has taken up the Semi-Slav, and although I don't think it suits his style quite as well, it's most probably for the move order reason. But there are players such as the OP who prefer not to play it, and they should definitely have their move orders well thought out.