Why is 1. d4 2. c4 3. b3 so bad for white? I recently played a game that opened like this:
1. d4 c6 2. c4 d5 { D10 Slav Defense } 3. b3 

 [FEN &quot;&quot;]

My third move, 3. b3 is apparently so uncommon that Lichess doesn't have it in its top 12 most common moves.
Stockfish rates the position after 2 moves at +0.2 and after 3. b3 at +0.0, so it's theoretically not terrible. However, the top 6 responses for black (topped by 3..Nf6 and 3..3Bf5) all have winning percentages favouring black (39%/54% for the knight), so there's clearly something wrong with it.
I often feel intuitively that I want to make this move (and in similar positions), reasoning that I prefer to bring my pawns towards the centre. Pawns on c4 and d4 look good, although I imagine b2 can become vulnerable.
OTOH, I can see that this move doesn't help develop any pieces, and maybe white is now going to be a move behind black after something like 3..Nf6?
I'd love some explanation about why this move is so dispreferred.
Stockfish evaluation is not the only criterion to determine whether a move is sound or not. The main issue with committing so early to 3.b3 (against this particular Black setup) is that there are no downsides to delaying that move.
In other words, even if you want to play some setup with b3, there's no reason do it right now. Instead, other moves that prioritize development like Nf3 should go first.
Anyway, just because a move isn't popular, it doesn't mean it's &quot;bad&quot;. It just means people prefer something else. Think for instance of 1.Nc3, a move that is hardly ever played. Is it &quot;bad&quot;? Not really. There are worse moves that get played more often. The reason 1.Nc3 is so rarely played is that there are more &quot;comfortable&quot; move orders from which you can get the positions that arise from 1.Nc3