After 3.a3, 3...b6 is fine as far as I know, but if you play 3...d5 you'll have a better version of the QGD as a3 is not especially useful, and you'd just have to play natural developing moves.
So not really, but you may need the QGD against the English!
The Mikenas attack if a variation you may want to avoid. Then, the QGD (1.c4 e6 2.Nc3 d5 3.d4 etc) is one of the best options you have.
It might be an idea to consider learning as black how to play against the fianchetto Benoni.
If white does not play d5, black playing d5 is a QGD while black capturing the d-pawn I think is a variation of the English. An early g3 by white can bypass the QID.
An early g3 by white can bypass the QID.
Many NID/QID players will simply go into the Bogo-Indian if White plays something like 3. g3 or 3. Nf3.
For example:
It's nice to have the trifecta in your repertoire: NID/QID/Bogo.
I like to play the Nimzo indian and Queens indian against 1.d4. Do I need to add the QGD in my repertoire? because there are some orders like 1.d4 Nf6 2.c4 e6 3.a3 ... if I play 3.b6 will tranponse to the petrosian variation? and I know, I can play 3.b6, but playing 3.d5 looks more effective according to the estatistics.
Thank you!