As title above, What's the best answer to 1.d4 Nf6 2.c4 e6 3.a3? Assuming that I, as Black, am a Nimzo-Indian Player, there is no hope for a Nimzo now. Would you go for 3...d5 (QGD with no particularly useful a3 move by White), or for a Benoni-like structure with 3...c5? (allowing 4.d5), or for a Queen's Indian structure with 3...b6? Or something else?
This really comes down to what you're comfortable with. Both the Benoni and the QGD are good options, precisely because of the reason you mention: a3 isn't really useful for White in those openings, so you'll be playing lines with a tempo up. According to chessgames.com, Black is already better. Which line (3... d5 or 3... c5) is better is just a matter of personal preference.
3... b6 is of course playable, and Black scores 50%, but it allows White to transpose to one of the main lines of the Queen's Indian with 4. Nf3.
According to chessTempo.com's database for over 2200 rated player games 3...c5 scores best for Black with 43% wins for Black and only 20% wins for White. By-the-way, there is another alternative you didn't mention which is 3...d6