What is the best reply to d4, d5, Nc3 ?

 Nf6 or e6

Nc6 .

WRONG!

Don't listen to this fool.  Post 2 is correct.  However, 2...e6 only works if you are willing to play a French Defense after 3.e4.

Otherwise, 2...Nf6 and then White has 3 main options.  3.Bg5 is the Veresov.  3...Nbd7 is theoretically best, but it requires studying.  3.Bf4 is the Jobava Attack and also has quite a bit of theory of late.  Lastly, 3.e4 is a version of the Blackmar-Diemer Gambit.  After 3...dxe4 4.f3, best is to take the second pawn, but you have to be a good defender.

yeesh calm down. nc6 is not exactly an error.

one response to consider, (that always gave me slight trouble) was 2.c5. It may not be objectively the best move, but it does test white's knowledge. to me its not even clear what white's best response is.

Nf6 would be safer to play, as e6 blocks the bishop on c8, and, after e4, your own pawn structure would be weaker than theirs, after them having gained control of the majority of the center squares. However, if you are an experienced player (unlike me!), and you know how to counter whatever they may put up against your e6 and d5 pawns, then you can go for the pawn development. 

Note: Most king's-pawn opening games play out with Nf6 at this point, as this not only protects the pawn, but also enables you to develop a piece in an organized fashion. 

1 d4 d5

2 Nc3 Nf6

3 Bf4

is Mark's Opening

Having been a Veresov player pretty much all of 2004 and 2005, I can tell you that 2...Nc6 is a positional error.  White has a significant advantage and is almost winning outright after 3.e4! dxe4 4.d5 Ne5 5.Qd6 Ng6 6.Bb5+ Bd7 (6...c6?? 7.dxc6 Qxd4 8.cxb7+ +-) 7.Nge2.

2...c5 may be ok, not nearly as bad as 2...Nc6, but I can tell you that it's a lot stronger after White has committed to 3.Bg5.

While I did say that 3...Nbd7 is theoretically best, and considered the main line, from a practical perspective, 3...c5 is one of White's biggest problems, and if you are a results based person, 3...c5 scored the highest at that point in time.  Black is taking a higher risk than 3...Nbd7, but I can tell you 3...c5 is one of the main reasons I quit the opening.

Nope - Already exists.  Assuming you are "Mark", this is nothing new.  It's the Jobava Attack.  It's a hybrid of the Veresov (B on g5 instead of f4) and the London System (N on d2 instead of c3).

2...c5 would be my answer as well. Seems intuitive enough, and the computer doesn't see anything wrong with it.

Looking closer, after 3.e4 dxe4 4. d5, it looks like White has a reversed Albin Countergambit with an extra tempo, but I'm not familiar enough with the Albin to say if that matters much practically.

I believe that 2...Nf6 is the best/most flexible move. as mentioned above 2...e6 may lead to a french defence.

2...Sc6 is definitely not that bad. as after 1.d4 d5, 2.Nc3 Nc6, 3.e4 again Black can transpose into a line of the French Defence with 3...e6. maybe not a very popular line, but far from being refuted.

son, that line transposes to the nimzowitsch defense with 3.nc3, a line i have played WAY longer than you have played veserov. for starters, your line is quite outdated.  3...dxe4 4.d5 nb8! 5.nxe4 c6! almost immediately equalizes. White can also try 5.f3!? and black can respond to this a million ways, all acceptable including accepting it, but e5, e6, c6 etc are all safer.

It was named Mark's Opening in 2008. Before that, no top grandmaster had ever played it regularly. I don't know who started incorrectly naming it the Jobava attack, or exactly when, but it was since 2008.

mark sounds lame, Jobava sounds gangsta.  instead of saying "im gonna clobber you on the chessboard" you can just say "im gonna Jobava you"

What about "I'm going to Mark you", in a psychological way by inflicting a devastating defeat on the opponent?

It should definitely be known as Mark's Opening.

who the heck is Mark?

 The person who invented the opening in 2008, as proved  here: www.Marksopening.blogspot.com

CURIOUS what you people think of 2...Bf5. I find myself playing that and setting up something like a London/ Slavish Type structure against 2.nc3. Waiting a little on Nf6 and doing c6 or c5 in one move. I find this confusing to some who play the Veresov like a system, waiting a lil to put the knight on F6 seems to confuse some. 

i know i have won at least one game playing Nf6.

and i have lost at least five games playing Nc6.(personal experience)

if you want to play safe i advise Nf6.