Moves played by beginners as black against the Italian opening

Dear forum, 

Regarding the italian opening I could use some advice. 

I'm learning the opening from the book "Fundamental Chess Openings" by Paul van der Sterren, and got quite familiar with the main line and possible variations. But however I encounter different replies from black after I play 3. Bc4 as white, like 3 ... d6 and 3... Qe7. In the Guico Piano mainline 3 ... Bc5 4. c3 I've also gotten 4 ... a6. 

How do I best approach these deviations from the main opening lines? And how do I train myself to seek for opportunities that are not covered in the mainline theory?

This however leads to a more general question for beginning players. How do I approach any devation in the first 4-5 moves from acceptable book moves?

I'm still looking for good books / resources on this topic. 

Best regards, 

Jos

This is for d6. I don’t really play Italian opening that much, so I used engine for best moves.

Deviations in the Italian suit the interests of white- most of those moves are simply a waste of a tempo. 
What are the ideas of your moves? What are the ideas of the Italian?
Quick development of the kingside, target the f7-square, take the center. You play c3 to support the d4 pawn advance to recapture with a pawn. Most of the time, unless you're straight up winning material, you will play the same way. c3, c4, 0-0, and simply develop. 

It looks like you've only played three games as white with 3.Bc4. That's not enough to really flesh out sidelines that you'll see- you just have to play more games. Again, in most sidelines you will play the same moves against. 

Best of luck. 

3... d6 isn't very good but it's theory and you should find it in opening books. Silly moves like 3... h6 and such can often be met by d4 exd4 and O-O or c3 If the Scotch Gambit proper is barely playable then it must be really good when Black wastes a tempo.

As a general rule of thumb, if your opponent plays something wacky that does not adhere to general opening principles, then just continue your own development, and open up the center favorably if your opponent wastes too much time.

You've raised an interesting point, which affects most club players. It seems to me there's scope for one of those openings book authors who produce so many books on variations major and mirnor to write a book specifically about the kind of sub-par moves we routinely encounter, eg.

Obviously it couldn't be comprehensive, but, given a sweep through the openings like that of Paul van der Sterren's book, it would be very useful to club players.

3...d6 is completely sound and carries fairly good flexibility. But black cant wing it, or you can get in serious problems. You can play it like an antoshin philidor with the bishop on c4, try something between a larsen variation of the philidor and some variations of the old steinitz with takes and g6, you can play it like a hungarian and play for be7, or you can play it with a more original flavor attempting to put pressure via bg4.  The only minus of this flexibility is that blacks' choices can be somewhat restricted by what white plays first. trying to force one variation into all scenarios (early c3,, qb3, c3 d4, 0-0 first, d4 first, nc3 early etc) usually leads to disaster or at least an inferior position in one of the variations.

But black must do his homework to know when to play each and move order matters. Esp with the early 4.c3 lines where aims for early qb3.