1.a3!!

a3 a6!!

Wow

2...e5 *yawn =

IMO the best way to play 1.a3 is just to play your black openings reversed. You already know how to play the openings and often a3 can turn into a useful move. For example: 1.a3, e5 2. e4, Nf6 3. Nc3 and black can't play a reversed Ruy. 

Sure you're throwing away white's advantage But outside of a Queen's gambit or Ruy (and maybe a couple other openings) you're pretty much doing that anyway. The practical advantage of knowing the opening better than your opponent should count for something. Engines will typically throw their entire opening book out the window very early and yet you're in positions you know and are comfortable in. 

3...c5 (black is threatening cxb4 which not only wins a pawn but I would argue the passed a pawn plus the other advantages is a virtually won game for black) 

4. bxc5, Bxc5 black is better. Better center and lead in development. 

4. b5, axb5 5. cxb5, d5 Black is crushing

4. Qa4, Nf6 black is at least =

4. Bb2 the only challenging move for white. Still 4... Nc6 looks equal. Protects the pawn. Can come to d4. Black is still threatening the b pawn. White's lack of KS development and lack of a center looks like black will have a strong initiative at the very least. 

Only if white wants a reversed Kan, which isn't very good for white.

Здравствуйте, есть ход a3 у белых. Но этот ход неправильный, и не даёт никакого развития. B3 намного лучше цель-слон на b2. На международных соревнованиях не тот и не тот ход не употребляется а это значит он не нужный.

VERY IMPORTANT TIP!

The move 1.a3 might be not that bad for guys like Magnus Carlsen, and even he plays like that only in blitz, but not for low-rated chess players. The main point is this: at a lower or even middle level you must learn to improve your game and play according to the "correct" rules. Such openings as 1.a3 will not teach the "correct", fundamental way of playing which is absolutely a must. The bottom line is: first learn to play very well, to play classically, to follow the fundamental rules, to know the basic and advanced classical structures, plans and maneouvres and when you achieve a high level, you might have fun with moves like 1.a3 but even then better in blitz.

Bb2 is best

Still, after Nc6 white has more space and will keep developing (even if he loses b4)

According to Leela ( The best chess player in the world that beats Stockfish in TCEC), she give a3 as 50% chance of winning , but she sees e4 and d4 as 55%.

Maybe this will help clear things up:

Hey I thought it was 1.a4!! 

But actually, the best move is 1.g4!!!!

According to Leela ( The best chess player in the world that beats Stockfish in TCEC), she give a3 as 50% chance of winning , but she sees e4 and d4 as 55%.

I am the opposite and can say for theory heavy players,  it's very possible to goad someone who plays 1.a3 into mainlines where ratings don't matter so much as precise moves so be careful