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If, for example, 3...g6, White continues 4.Bg2 (4.d3 Bg7 5.Bg2 is the same) 4...Bg7 5.d3. Now Black has 5...e5, playing this before 5...Nf6 in order to continue 6.O-O Ne7. The sequence 5.O-O e5 6.d3 is a transposition.
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Another possible line is 3...Nc6 4.Bg2 e5 5.d3 (5.O-O Nf6 6.d3 is the same) 5...Nf6 6.O-O The disposition of the White pieces is the same as the previous line. Now Black can continue 6...a5 7.e3 or 6...Nd7, when the Knight is headed for c5.
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Also possible is 3...c5, and now White challenges the center immediately with 4.e3 or 4.b4.
The idea of challenging the center can be played a move earlier. The most direct line is 3.e3 Nc6 4.exd4 (4.Nxd4 Nxd4 5.exd4 Qxd4 is the same) 4...Nxd4 5.Nxd4 Qxd4 6.Nc3, planning 7.Nb5 Qb6 8.d4. Black can answer this with 6...c6 7.d3 Nh6 and ...Nf5, or with 6...e5.
Another idea is 3.b4. After 3...g6 4.Bb2 Bg7, White has 5.e3, 5.g3, or 5.d3. If 4.d3 Bg7 5.g3, Black can bolster the center with 5...e5 6.Nbd2 Ne7. Another interesting line is 4.e3 e5, when 5.exd4 is more solid than 5.Nxe5 dxe3 (5...Bxb4?? 6.Qa4+) 6.dxe3 (6.fxe3 Qh4+ 7.g3 Qe4) 6...Bxb4+.
After 3.b4, Black can also try 3...f6 4.e3 e5 5.Qb3, or 3...c5, when White can ignore the attack on the b-Pawn with 4.Bb2 or 4.g3.
