Chess Opening Theory/1. e4/1...c5/2. Nf3/2...Nc6/3. Nc3/3...d6

3...d6
This is the main line and is a non-committal move. Black retains flexibility to wait and see if white opens up the game with d4 or opts to keep it closed such as d3 and maybe c3 to prepare the hallmark d4 push and locking up the center.

Theory table
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1.e4 c5 2.Nf3 Nc6 3.Nc3 d6