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

=Queen's Gambit Declined=

3.Nc3
After building a pawn center with their first two moves, White begins the development of their minor pieces with 3.Nc3. On c3, the knight controls the d5 and e4 squares. White would like to play e4 at some point to gain even more presence in the center, and 3.Nc3 facilitates this break.

White's dark-squared bishop will likely go to g5 after Black plays Nf6.

Theory table
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'''1. d4 d5 2. c4 e6 3. Nc3'''