Hello!
I am a Class A player and started to play the Ruy Lopez two years ago to improve my strategic abilities. (I played Bishop's opening and King's gambit before)
Most of my opponents played (or transposed to) the Modern Steinitz with 5...Ld7 or
deviated already at move three,
often because they don't like the Exchange Variation.
Only a few played the main line (3...a6 4.La4 Sf6 5.0-0 Le7 6.Te1 b5
7.Lb3 d6 8.c3 0-0 9.h3) They all continued with the Chigorin or the Zaitsev.
Nobody played the Arkhangelsk or the Moeller.
Nobody played some rare systems with an early Lg4.
Only one opponent tried the Marshall.
Only one opponent played the Open Defence.
Fourth move deviations ( except the Modern Steinitz) have been very unpopular
because of the Exchange variation, but there were two exceptions:
1) One opponent provocated and confused me:
4...b5 (he wanted to avoid some Delayed Exchange Variations and to gain time on the clock) 5.Lb3 Sf6 6.Sg5?! (6. 0-0 is good and probably transposes back to the main lines) d5! 7.exd5 Sd4! ...
2) Another switched to 3...a6 4.La4 f5 after some losses against 3...f5 4.De2 in a Blitz session.
Since Khalifman's books are your main source I want to make a few comments from the perspective of a Class A - player:
Take the rare lines (chp 14) seriously!!
3...d5 was once played against me in a serious tournament game ??
3...g5 tries to drive away the knight, of course.
3...Ld6 probably tries the regrouping Sf6, 0-0, Te8, Lf8,
which is by no means bad in the Spanish Four Knights,
but too slow in the Lopez after energic play.
3...Df6 was investigated by Gunderam in the Sixties.
Be careful and don,t play 0-0 too early.
3...Lb4 tries to trick White into an improved version of
the classical defence.
3...Se7 most likely transposes to the Modern Steinitz, if you follow Khalifman's suggestions.
It is played frequently because it is a way to get there and avoid the
Exchange variation.
Since it was recommended in a recent SOS (Secrets of Opening Surprises),
maybe you will face it soon.
3...g6: Khalifman's gambit (d4, c3) may be dangerous, but I think it is very unpractical, because 3...g6 is indeed very rare.
There is nothing wrong to play in Lopez-style with c3 and d4, play will most likely transpose to the Modern Steinitz or some line of 3...Se7.
3...Sd4 is a rare Bird indeed, but black succeeds in getting an unusual position.
Khalifman made the serious mistake to repeat it against Kasparov after he achieved a draw . Have a look in his book to see the opening phase of their second game.
3...d6 is surprisingly popular. This is a passive defence, but very solid. The very young Karpov played it with some success! For a first reading I would skip Khalifman's chp 17 on the Old Steinitz and study chp 23 instead (3...Sf6 4.0-0 d6) This move order avoids dangerous white setups with 0-0-0 and is very common.
Classical Defence 3...Lc5
If black wants to get the Berlin Classic (Sf6 and Lc5) it is very attractive to start with 3...Sf6. Since many people are afraid to play the Berlin Endgame, they make an early concession like 4.d3 or 4.De2. So I guess if someone starts with 3...Lc5 he has something special in mind.
Maybe
1) agressiv: 4.c3 f5 combined with 4.0-0 Df6.
From my own experience as black both lines give good practical chances against an unprepared white player, especially in blitz or rapid games, but black must know his stuff well. I think this approach is not popular, because people enjoying sharp lines would go for the Jaenish.
2)more solid: 4.c3 Se7 (this was suggested in a recent SOS) combined with 4.0-0 Sd4.
Since 4...f5 is really messy, perhaps it would be more practical to start with 4.0-0 against 3...Lc5. But then Df6 gains in strength, if white plays this line passively he could be worse very quickly.
To sum up: after Khalifman's 4.c3 study 4...f5 carefully,
if you choose 4.0-0 study 4...Df6 carefully.
Jaenisch Gambit 3...f5.
4.Sc3 is maybe the best move, but very impractical for an amateur. If someone adopts the Jaenisch, it is his main defence against 1.e4. So at our level he certainly knows the theory much better than you, which is a very big advantage in this lines.
4.d3 is a good and practical alternative, but since it is such a natural move your opponent will have much experience with this one, too.
4.De2 is an underrated offbeat try. Jaenish players usually don´t like to start their own game on move four
4.d4 results in a piece-sacrifice, maybe it is worth a look?!
Berlin Defence 3...Sf6
Don't play moves like 4.d3 or 4.De2, imho it is too easy for black!
Study careful the Berlin Classic Sf6 & Lc5, it is seldom but critical.
If you are afraid of the Berlin-Endgame, then play
4.0-0 Sxe4 5.Te1 Sd6 6.Sxe5 Le7 7.Lf1 as recommended in
"Kaufman, The Chess Advantage in Black and White"
Since Kaufman suggests the Berlin Endgame as blacks main defence against 1.e4, I would take his advice seriously and try to avoid this endgame. It has little to do with standard Lopez play and is difficult to play for white, so I would not play it as a Lopez starter. (I adopted the Berlin Endgame with black, so I play it with white just to learn, but my white results are poor.)
7.Ld3 is an aggressive alternative. It was discussed in the famous match Steinitz-Zukertort. Steinitz defended accurately, but maybe your future opponents have not the defensive skills of Steinitz.
I hope my post helped you a little.
I will post some comments to the variations after 3...a6 soon.