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

3. Nc3
Most of the time in the Sicilian, White puts his Knight on c3. White may yet play d4, but for the moment the question is yet to be answered. White's second move suggests the possibility of NOT playing d4 (avoiding the exchange), and playing a closed position instead of the slashing, attacking types of position, that are more common to the Sicilian Defence. Leads to 3.g3 g6 4.Bg2 Bg7 5.d3 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