3. Nc3 in the Slav

I encountered a tournament game recently where I was playing the Slav as black, and it went like 1. d4 d5 2. c4 c6 3. Nc3 dxc4...

I believe the usual response goes like 3. ... Nf6 4. Nf3 transposing into mainline Slav, however I remember reading somewhere that 3. Nc3 instead of 3. Nf3 is "dubious", although I forgot the reason why.

Could someone please enlighten me?

3. Nc3 in the Slav is far from dubious, it's one of the most common lines.  The "problem" is you then have to learn the 1. d4 d5 2. c4 c6 3. Nc3 e5!? line, (the Winawer Countergambit I think is the name, I've never faced it since 3. Nf3 completely avoids it).  No reason to bother learning that fairly sharp line... Just play 3. Nf3 and carry on with standard Slav stuff!

Agree with Connor. Here is a whole pile of information on it:
http://www.chessgames.com/perl/explorer?node=416823&move=3.5&moves=d4.d5.c4.c6.Nc3&nodes=10703.75972.75973.159605.416823

Both replies are perfectly fine and both avoid certain variations. PersonallyI play 3. Nf3 to avoid the Noteboom variation as 3. ... e6 can be answered with 4. Nd2.