Chess Opening Theory/1. d4/1...d5/2. c4/2...e6/3. Nc3/3...c6

In this position, by delaying Nf6, Black keeps many opportunities open. They can keep on playing in the normal Queen's Gambit Declined (QGD) fashion with 4...Nf6 (whether White plays e3 or Nf3), or can try other options.

4. Nf3 can lead to regular QGD games or alternatively the Noteboom variation: 4...dxc4. Here, Black aims to get two passed pawns on the flank in exchange for White's big center. The middlegame is critical, but if Black survives, the endgame will be in their favor.

On the other hand 4. e3 can lead to a stonewall defense position with 4...f5, where Black's usual plan is a quick attack on the king side and can often involve a rook lift to f6.

Another option for White, though less popular, is the Marshall Gambit of the Semi-Slav, with 4. e4. This move aims at exploiting Black's lack of control over the e4-square, not having played Nf6. The main line continues 4...dxe4 5. Nxe4 Bb4+ 6. Bd2 Qxd4 7. Bxb4 Qxe4+ with a sharp game. Usually White blocks with Be2 and leaves the g2 pawn hanging. Black can capture it, but not the rook (Bf3 wins the queen). In return, White can have lasting pressure and tactics involving the d6 weakness. An example game played in this line is Bacrot - Morozevich, Biel Rd2, 2012^[1].

1. d4 d5 2. c4 e6 3. Nc3 c6

Wikipedia has related information at Semi-Slav Defence.