Chess Opening Theory/1. e4/1...e6/2. Nf3/2...d5/3. Nc3

This is a relatively rare sideline of the French defence, and for good reason. White's approach is not particularly ambitious, which gives Black a relatively smooth game. Black has two main responses.

3...Nf6 is the main line. This allows White to push 4. e5 and after 4...Nfd7 5. d4 c5 Black gets a fairly thematic French structure, while White's knights are in the way of the pawns. Similar positions can be reached from the Steinitz variation where White plays f4 instead of Nf3, or the Advance variation where White can keep their centre together with c3.

3...d4 is Black's main alternative. This results in more open positions because after 4. Ne2 c5 5. c3 White is able to break apart Black's centre. Black can't hold onto their pawn and should instead prioritize development with 5...Nf6 or 5...Nc6.

1. e4 e6 2. Nf3 d5 3. Nc3

Wikipedia has related information at French Defence.