With the move 3. Nc3, White continues development and puts pressure on the d5 pawn, and also controls the e4 square for a possible e4 break in the future. White wants to play e4 at some point in the future, and the move 3. Nc3 helps with this break.

3...Nf6

Black continues development, defending the d-pawn and controlling the e4 square, stopping white from playing e4 himself. However, this move also blocks the queen's protection of g5, allowing White to actively develop the dark squared bishop with 4. Bg5, pinning the knight on f6.

3...c6

With this move, black also defends the d-pawn a second time, but also controls the b5 square, allowing black to play dxc4 and then b5 in the future. White can allow this with 4. Nf3, entering the sharp Noteboom variation, but could also make the game complicated with 4. e4!?, going into the Marshall Gambit in the Semi Slav, in which White gambits a pawn for the bishop pair and threats against Black's king. 4. e4 takes advantage of the fact that 3...c6 doesn't do much for Black's development, and doesn't control e4.

3...c5

This is the Tarrasch Defence, in which Black immediately strikes at the d4 pawn, aiming to create an aggressive bid for central space. Black usually accepts an isolated queen's pawn, in exchange for a freed light squared bishop and active pieces.

3...Be7

This is the Charousek Variation (also known as the Alatortsev variation). With this move, Black continues development of the kingside, but by delaying the development of the knight to f6, he also makes Bg5 impossible for White, meaning white's dark squared bishop usualliy ends up on f4, leading to very different kinds of positions compared to 3...Nf6

3...Bb4

This is known as the Accelerated Ragozin, in which Black aims to get a sort of Nimzo-Indian/Ragozin type position. However, the drawback is that when white plays a3, and Black captures on c3, White gets an improved Samisch Nimzo-Indian, as White can easily undouble his pawns with an eventual cxd5 in the future.

3...a6

This is the Janowski Variation, which has gained popularity in the last 5 years. Here, Black threatens to take the pawn on c4, and then hang on to it with b5, similar to 3...c6, which compels White to play 4. cxd5, though like 3...Be7, White isn't able to develop the bishop to g5.

For explanation of theory tables, see theory table and for notation, see algebraic notation..

1. d4 d5 2. c4 e6 3. Nc3

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Wikipedia has related information at Queen's Gambit Declined

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.