What do you play against d4 d5 e3?

it blocks the pathway of c1 bishop, but i see this opening fairly often at amateur levels.

i always played 2...bf5 and go for the london system as black, but the bishop on f5 is unstable and often becomes a target for white, and it can even become a liability when dealing with king side attack.

so im currently planning to change the response to it to 2...nf6 and then trade off the light squared bishop via b6ba6.

what do you play against it?

Per the database, I've faced this as Black 33 times on Lichess, and played Nf6 28 times with a 46/21/32 W/D/L rate.

I think 2...c5 has significant merit as well, having said that.

You can play c5 (break the pawn chain aggressively) , as well as e6 followed by c5 , where c5 is protected by bishop and you have solider pos

The general advice when facing crap openings is to keep calm, follow the standard opening principles and enjoy the extra freedom a regular opening would not give you. In this case, one possible approach would be to handle it like London, except the bishop is inside rather than outside (which should be an advantage for black). But it all depends on what white plays next.

@mkubecek said in #4:

The general advice when facing crap openings is to keep calm,

i dont think the opening is necessarily crap. both colle and stonewall, opening that can arise from d4 d5 e3, has its own upside compared to the london.
although i do agree that both are theoretically inferior than london tho

I play Nf6 instead to defend the d5 pawn

How does Nf6 attack the d4 pawn?

The easiest response to Colle/London systems IMO is hedgehog structure, because it prevents white from putting the Knight on e5, and so changes the game to where you won't be in danger of some fast attack if you make an imprecise move. It tends to take these players out of their comfort zone because Ne5 ideas are critical in main lines.

Further, when the position simplifies, black has a very straightforward plan with minority attack in a lot of endgames, while it is harder for white to find a clear plan.

Overall, white only has a small edge, and the position is quite fluid where white has to actually outplay you to win, not use some fast attack/pawn storm they learned somewhere.

Some of my example games, just to show the structure and piece development for black. Obviously will vary with white's setup as well, but the structure should be roughly the same.
It may not show it, but black can play Qc7-Qb8-Qa8 to get better center control in many cases, if there's nothing better to do.

usually I play the standard 2...Nf6, 3...e6, 4...Be7, 5...0-0
usually White plays 3.Nf3 4.c4 5.Nc3 6.Be2/3

then 6...dc 7.Bxc4 c5 with a position similar to, but better than (for Black) main-line QGA.

The Colle System is nothing to fear.