As title above, how to play vs.
[fen ""] 1.e4 e5 2.Nf3 Nc6 3.Bc4 d6
I see that this move (3...d6) is generally considered inferior in books, or not even mentioned in books about 3.Bc4, but I cannot see a clear way to obtain advantage.
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The game will get the characteristics of the Philidor Defense rather than the Italian Game. Wikipedia calls it the Semi-Italian Opening.
I. A. Horowitz called the defence "solid", also writing: "It does not seem quite sufficient for equality."
The Wikipedia article notes several possible continuations for White, with 4. d4 probably the best bet for advantage.
1.e4 e5 2.Nf3 Nc6 3.Bc4 d6:
365chess.com database shows:
MOVE | games | year | White + Draw + Black -------+-------+------+--------+--------+------- 4. d4 | 491 | 2018 | 50.3 % | 24.4 % | 25.3 % 4. c3 | 359 | 2018 | 53.5 % | 20.6 % | 25.9 % 4. d3 | 357 | 2018 | 45.4 % | 22.7 % | 31.9 % 4. h3 | 323 | 2016 | 42.7 % | 25.7 % | 31.6 % 4. Nc3 | 289 | 2018 | 50.9 % | 16.3 % | 32.9 % 4. O-O | 249 | 2019 | 50.6 % | 22.1 % | 27.3 %
White will get a space advantage and freer development after d4 (whether delayed or played immediately). Or White can play the pawn to d3 and the fact that his Bishop is outside the pawn chain (and Blacks Bf8 is inside) gives White an easier position.
There is no knock-out blow for White as Black is accepting a slightly passive but solid position.
There is no way to "crush" that move. You just play! Develop your pieces, get space in the center and obtain a small advantage. Your opponent won't be losing a piece or something like that.
I assume 3...d6 may have some ideas related to a pin on g4 (for instance, 4.d4 Bg4) A c3 pawn advance can be useful to prevent some ...Nd4. I think you'll be fine after 4.0-0 Bg4 5.c3, but also 4.Nc3 Bg4 5.h3 or many other sequences
In short, I don't know what the best answer is. I doubt anyone in the world actually does. Analyze your games and find what works best for yourself