Thank you all for your answers.

I think that against the Slav I'll go with 3. Nf3 in order to avoid the Winawer Counter-gambit (3. Nc3 e5), and because black will be less inclined to take early on c4.

And against 2... e6, I just discovered the Exchange Variation (1. d4 d5 2. c4 e6 3. Nc3 Nf6 4. cxd5 exd5 5. Bg5), and I like it.
Plus white scores tremendously well with it. Taken from an opening explorer : white won 49.3% of the time, black won 19.4% of the time, and 31.3% of the time it was a draw. If we compare that with for example the stats of 1. e4 : white won 39.2% of the time, black won 32.6% of the time, and 28.2% of the time it was a draw. The difference is just astounding.
So I'll definitely answer 2... e6 with 3. Nc3.


I just have three more questions!


Firstly, what is the difference between playing 3. Nf3 and 3. e3 in the Queen's Gambit Accepted (1. d4 d5 2. c4 dxc4) ? 

I've already made some searchs :
3. Nf3 is three times more popular than 3. e3.
But, 3. e3 scores much better for white than 3. Nf3 :
- With 3. Nf3, white won 38.5% of the time, black won 24.2% of the time, and 37.3% of the time it was a draw.
- With 3. e3, white won 41.9% of the time, black won 21.9% of the time, and 36.3% of the time it was a draw.

And from what I've seen, it looks like 3. e3 allows black to play 3... e5, while 3. Nf3 allows black to play 3. Nf3 Nf6 4. e3 Bg4. No idea which of these two moves I should fear more though.
And I'm not sure if 3. e3 or 3. Nf3 allow white to avoid other things that I didn't see..


Now for the second question : In the Exchange Variation of the QGD, what is the difference in the move order between e3, Bd3, and Qc2 ?
Sometimes it's e3, then Bd3, then Qc2. Sometimes it's Qc2, then e3, then Bd3. And sometimes it's e3, then Qc2, then Bd3. The three of them are going to be played anyways, but for some unknown reasons white seems to generally score better by playing Qc2 first...


And, the third question : With white, is it possible to play the Exchange Variation of the QGD all of the time? This without allowing black's light squared bishop to go to f5, since I want my light squared bishop to go to d3.
And if it's not possible, how can we play it the most frequently as possible?

Because after 1. d4 d5 2. c4 e6 3. Nc3 c6 (Semi-Slav) I don't see how we can transpose to the Exchange Variation.
If 4. cxd5 exd5 5. Nf3 black will play 5... Bf5.
If 4. Nf3 I can only pray black won't play 4... dxc4. And even if he plays 4... Nf6, he will easily be able to put his light squared bishop on f5 because Qc2 (trying to prevent the direct ... Bf5) can be answered by ... g6 and then ... Bf5.

While, after 1. d4 d5 2. c4 e6 3. Nc3 Be7 (Charousek Variation), I have a book where the author wants to play 4. cxd5 exd5 5. Bf4 (white scores well in this variation too, though not as much as in the Exchange Variation with the bishop on g5). But I wanted to find a combination of moves which would lead white to the Exchange Variation. I searched for hours for something beginning with 4. cxd5 exd5 5. Nf3 or 4. cxd5 exd5 5. Qc2, but didn't find anything.
But, after persevering, I believe I finally found the answer : 4. Nf3 Nf6 5. Bg5 O-O 6. cxd5 exd5 7. e3. (and black can't play 7... Bf5? because of 8. Qb3) And I have the Exchange Variation.
According to an opening explorer, 4... Nf6 will (almost) always be played by black. While at the 5th move, black can also try 5... h6 but after 6. Bh4 black will then always play 6... 0-0 followed by 7. cxd5 exd5 8. e3, and black can also play 5... Nbd7 but then I have no problem achieving the Exchange Variation.
6... Nxd5 is one of the two only move that could prevent the Exchange Variation from happening but it's not that powerful and is only played 14% of the time, the second move is 5... c6 but it is only played 4% of the time.


Again, thanks in advance for your answers.

3.Nf3 also avoids the Hennig-Schara gambit, or whatever it's called.

Oh, and one more thing: also against the QGD, it depends on how you want to reply to the triangle. Just like against 2...c6, 3.Nc3 allows you to play the Marshall, while 3.Nf3 gives you more options if the game continues 3...c6 4.e3 Nf6.

Dan Heisman covered the Slav/Nc3 question briefly in one of his Novice Nook columns at ChessCafe.com. Look for the column called "The Most Common Opening Inaccuracies" (March 2006), which can be found here:
http://www.chesscafe.com/archives/archives.htm#Novice%20Nook

and of course if white wants to play the Catalan he has to go 3.Nf3

Hello.

I was just wondering, in the Queen's Gambit Declined, after 1. d4 d5 2. c4 e6 (or 2... c6), what is the difference between playing 3. Nc3 and 3. Nf3 ?

I looked in an opening database/explorer :
- After 2... e6, 3. Nc3 is 3 times more popular than 3. Nf3, but 3. Nf3 scores better for white (+43.6% -22.4% for 3. Nf3, versus +42.6% -26.1% for 3. Nc3).
- After 2... c6, 3. Nf3 is 1.5 times more popular than 3. Nc3, and 3. Nf3 also scores better for white (+40.9% -22.5% for 3. Nf3, versus +41.2% -26.3% for 3. Nc3).

Is there really a big difference? Isn't white just going to play both Nf3 and Nc3 anyways?
Which of those two moves do you recommend me to play first? Should I play 3. Nf3 just because it scores better than 3. Nc3 ?
Do each of those two moves allow white to avoid specific variations? If yes, what are those variations?

Thanks in advance for your answers.