Chess Opening Theory/1. e4/1...e6/2. Nf3/2...d5/3. e5/3...c5

This is Black's most natural and common move.

White usually plays this variation to respond with 4. b4, the Wing gambit. While not objectively the best move, there aren't many reasons to use this move order to transpose into different French lines.

4. d4 transposes into the Nimzowitsch system of the French advance variation.

4. c3 is not an effective way to get into the advance variation, since Black can play the move 4...d4, resulting in completely different positions.

1. e4 e6 2. Nf3 d5 3. e5 c5

Wikipedia has related information at French Defence.