a3 has only one logical followup: b4. If black plays a5 or c5, your idea is toast. a4 is a much better opening move because it cant be directly circumvented immediately.
What if someone's first move is a3

ok but there is no point to go for a grunfeld reverse where a3 is useless.
U have to find a way to get a variation where control of b4 can be important. For example, someone talked about Benko gambit reversed...
So you suggests 1.a3 c5 2.Nf3 d5 and then 3.c4!? d4 4.b4 or something?
Are you perhaps thinking of the 1980 game that began 1 e4 a6 ?

a3...?!
he is the first overconfident Master or an innocent beginner!
what do you mean? Are you implying that starting with an A3 won't work?

1. a3 is fine. Whether u play it or ur opponent does. Under 2000 chess losing teensie-tempos early ?....really doesn't mean anything, since most games, slow or fast, are settled w/ a one-move tactic or piece-hung zinger or humdinger or eye-popping blinger....later on.
Ok. Selfie this. Do u feel that AK bolted up rite after AM played it against him ?....of course not. AK probably just kinda giggled & moved on. It didn't throw him off. He's too much the professional to have gotten jiggy & off the beam.
I've heard old skool teachers say that this is a big No-No to their young students. No it isn't. It's just dubious & kinda doesn't do anything right away. 1. a3 may be turn out to be a huge move that u don't need to make later on 'cuz u already did !....right ?
I say play it if u have a exploring heart. Then play it again if it felt good the first time. I mean, that's why some ppl need a rib cage, right ?
hmm.. I guess that's a good idea..

It's called Andersson's Opening. Basically it's whites way of saying he wants to be black. Also it opens up the queenside bishop along the a2 g8 diagonal and stops black from playing things like Ruy Lopez.
wait what?!

a3 has only one logical followup: b4. If black plays a5 or c5, your idea is toast. a4 is a much better opening move because it cant be directly circumvented immediately.
Pretty wrong.
Surprising but the idea of a3 is not to continue with b4...

ok but there is no point to go for a grunfeld reverse where a3 is useless.
U have to find a way to get a variation where control of b4 can be important. For example, someone talked about Benko gambit reversed...
So you suggests 1.a3 c5 2.Nf3 d5 and then 3.c4!? d4 4.b4 or something?
Something like that yes.
But any system where b4 can be good at some point (and not at move 2, who would like to play a Sokolsky ).
Benoni, Benko, KI( pointed out above), and i guess we can find other systems.

There is recent book out titled Play 1...d6 Against Everything. It seems to me a person could use 1 a3 followed by 2 d3 as white, use 1...d6 as black, and therefore have their entire repertoire in that one book. Sounds pretty good huh. However, the book's repertoire is based on the Philidor and Old Indian, so you have to be willing to play rope a dope and count on getting your punches in later.
One perfectly good antidote is 1. ...g6.
White can then aim for a reversed KID Panno.