Chess Opening Theory/1. d4/1...d5/2. Nf3/2...Nf6/3. Bf4

= London System with an early Nf3 before Bf4 =

Starting with the moves: 1. d4 d5 2. Nf3 Nf6 3. Bf4, this line diverges slightly from the typical modern London System by not placing the bishop immediately to f4 to establish the pyramidal pawn structure with c3-d4-e3. For a general introduction to the London System, take a closer look at 1. d4 d5 2. Bf4.

After Black develops to d5 and Nf6, White plans to place their King's knight on f3 and pawns on e3 and c3 to strengthen their centre. It can be used against nearly all of Black's common responses to 1. d4, including 1...d5. From this position, Black usually plays either 3...c5, 3...e6, 3...Bf5, 3...c6, 3...g6, or 3...Nc6.

Advantages for White with an early Nf3

 * 1) Quick Development: By deploying the knight and Bishop rapidly, White can achieve a lead in development. If desired, this allows for speedier control over central squares and a faster kingside castle.
 * 2) Flexibility: The moves played do not commit White to any particular pawn structure early on, leaving room for various plans, such as an early c4 to challenge the center or a quick e3 to solidify the center pawn.
 * 3) Avoids Early Tactics: In some lines against the London System, Black can opt for Nh5 attacking the bishop when e3 and Bd3 are played. In this setup, such tactics are unavailable.

Drawbacks for White

 * 1) Misses the Typical Solid Structure: One of the main advantages of the traditional London System is its solid and consistent pawn structure on c3-d4-e3. White may not have time to establish this structure in this sequence, especially if Black responds aggressively.
 * 2) Qb6 Counterplay: After 1. d4 d5 2. Nf3 Nf6 3. Bf4, as you rightly pointed out, Black can opt for ...Qb6. This move attacks the b2 pawn and puts pressure on the d4 pawn. Without the supportive pawn on c3, White may have to make some concessions to protect their assets, such as Qb3 or b3, which could weaken the queenside or lead to doubled pawns after Qxb3 axb3.
 * 3) Potential Isolated Pawn: If White eventually pushes c4 to challenge Black's d5 pawn and exchanges occur, White could find themselves with an isolated d4 pawn, which could become a weakness in the endgame.

Theory table
. '''1. d4 d5 2. Nf3 Nf6 3. Bf4'''