Hi, 
thanks for all your answers and thoughts! I`am now sure that 3....Be7 is better. I like to play always 3...Be7 and just `be a sitting target in the QD`because I`am knowing that this defence (say on the way to the Tartakower) ist sound and understanding is more important than anything.....
Now I´am thinking what is better after 1.d4 d5 2.c4 e6 3.Sc3 Be7! 4.cxd5 exd5 5.Bf4 c6 or 5...Nf6. Aronian and others play 5...c6 but perhaps 5....Nf6 is a little more flexible and there is more chance for an interesting battle. What do you think? (Sorry, once again.... )

greetings

wolfsblut

Definitely- and the following players have favoured this move order: Aronian, Karpov, Kasparov, Anand, Nakamura, Geller..........I don't think that any of them have played 3..Nf6.

A QGD player might only want to play the QGD complex and not learn the Nimzo-Indian. But learning both QGD and Nimzo might be better than settling for the 3...Nf6 line and not learning another opening. That endgame line as well as the old main line where White plays Bd3/Nge2/f3 look unpleasant enough to warrant learning Nimzo or Ragozin to avoid those lines.

What are the alternatives to the endgame line except the old main line? I think this is the reason to play 3...Be7 instead as well.

With the move order 1. d4 Nf6 2. c4 e6, the option of the Ragozin is either the advantage or the disadvantage of this move-order; if the QGD player only plays within the confines of the QGD and does not play the Ragozin, the transposition to the 3...Nf6 line is inevitable with this move order.

You know, there's also 3...Nf6 4.cxd5 Nxd5, heading for a Semi-Tarrasch.  Nothing wrong with it and you might look there if you want to avoid the regular Exchange lines.  I know of no way that White can profitably avoid transposing to a regular Semi-Tarrasch.  It's no cakewalk for Black but certainly playable.

What i really meant is that a flexible QGD player should consider learning 2-3 lines and Tarrasch or even Semi Tarrasch (playing Nff6 and then ...c5 and capturing with the Knight on d5 as GM Athanasios Mastrovasilis does frequently) is actually two of them. Being a sitting target with an inflexible repertoire is not a good thing in the QGD. 

But again it is a matter of choice. Cox's lines in the ...Nf6 exchange are fine and give the opportunity to play later other lines than the traditional ones of the QGD (Ragozin for example). 3...Be7 seems also strong these days and if someone works on it he will have many benefits. It is a choice of taste, no clear answer exists in my humble opinion.

I think the real question is this: Is 3...Be7 better than 3...c5?

Yes.

The Exchange Variation with 3...Nf6 I find has serious problems and I would probably play 4. cxd5 instead of the Catalan if all my opponents played 3...Nf6. I read John Cox's book on the QGD, and the endgame line with 3...Nf6 seems very difficult to win to me.