I play the Exchange variation as White when Black plays 2...e6 against the Queen's Gambit. Should I play 3. cxd5 to avoid the transpoition to the Slav 3. Nc3 (hoping for 3...Nf6 4. cxd5) c6!?
[FEN ""] 1. d4 d5 2. c4 e6 3. cxd5!? (3. Nc3 c6!?)
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After 3.cxd5 exd5 4.Nc3, if black plays 4....Nf6 or 4....Be7, then white indeed succesfully transposed to the mainlines of the QGD and avoided any transpositions to the Slav.
Black's best reply is probably 4....c6, after which white has 3 good options: 5.Nf3, 5.Bf4 and 5.Qc2.
In all variations, black has a good version of the QGD, as white cannot prevent him from developing his bishop to f5.
It seems that after 3.cxd5 black equalizes rather easily and that 3.Nc3 or 3.Nf3 are much more principle moves. However, 3.cxd5 cannot be called a mistake. If the Carlsbad structure suits your playing style, then it is worth a try.
[StartPly "4"] [FEN ""] 1.d4 d5 2.c4 e6 3.cxd5 (3.Nc3 Be7 4.cxd5 exd5 5.Bf4 c6 6.e3 Bd6) exd5 4.Nc3 c6 (4...Nf6)(4...Be7) 5.Qc2 (5.Nf3 Bd6 (5...Bf5)) (5.Bf4 Bd6 (5...Bf5)) 5...Bd6 (5...g6)
It is basically a trade-off: are you so afraid of the Slav that you're willing to play a slightly inferior Exchange line?
The main line of the QGD Exchange is:
[FEN ""] 1. d4 d5 2. c4 e6 3. Nc3 Nf6 4. cxd5 exd5 5. Bg5
If you have a clever opponent, they can play Be7 instead of Nf6 if you exchange too early:
[FEN ""] 1. d4 d5 2. c4 e6 3. cxd5 exd5 4. Nc3 Be7
In that case, you can only play variations with Bf4, which promise less chances of advantage.
Exchanging on move three is too early.
After 1 d4 d5 2 c4 e6 3 cxd5? exd5 4 Nc3 Black solves his main problem of how to develop with the Queenside Bishop. This achieved with with 4 ... c6! and ... Bf5!, for example:
[FEN ""] [StartPLY "6"] 1. d4 d5 2. c4 e6 3. cxd5 exd5 4. Nc3 c6 5. Qc2 Bd6 6. Nf3 Ne7 7. Bg5 Bf5
The move 3 cd5 is certainly not good as it does nothing to improve White's position, and in fact helps Black's development opening the c8-h3 diagonal.
On general principles, trading a slightly wing pawn for a center pawn is good, but on the other hand, the e6-pawn no longer blocks Black's queen bishop. cxd5 may allow the White bishop to move a bit more, but it's to b5 (Black is playing c6 anyway).
White is trading an aggressive pawn that influences a center square on Black's side of the board for a slightly more passive one.
Also, if White is trying for a minority attack, this shows his hand.
cxd5 isn't bad per se--if you are playing a higher-rated player and need a draw, it may be a throw down the gauntlet moment to say "you'll have to take risks to beat me!" But I usually find it's best to keep the tension going. This is oversimplifying, but sometimes chess is a game of chicken, and if you take to relieve a bit of pressure on you, you may relieve more on your opponent.
The Objective of White is to develop the Pieces and attack Black ASAP. Both sides should try to complete Development and then Play in Center/Flanks .
When White does a cd5 then it is helping Black to free himself and Develop the c8 Bishop . You know Bishops are already active from their Initial Position & cover the Squares that come across its Diagonals . So it is not a very good Strategic Decision from White to play cd5.