I think this depends on what you want to achieve with Black. As I am a classical guy, I would instantly play 
4. Nf3 Bg4 5. Be2 
and proceed from there. Just develop your pieces and have a small edge in space. Black will at some point take on f3 and play e6, trading the two bishops for a solid structure. If that's all you are aiming for and are happy to play with Black, then a6 might be right for you (: 
It's just one of those notorious += positions that Black can perfectly play but White does not need to think a lot to get an edge. 
It reminds me of the Modern with c6+d5, where the dark-squared bishop lands on g7 not e7, which should be an improvement for Black. So, as Black I would rather go 
3. Nc3 g6
if you want that kind of game.

Be reminded though that 3. Nd2 cuts out all these shenanigans!