LeeRoth wrote on 04/17/20 at 22:18:21:
Well, 3.e3 can’t be too bad, as a lot of strong GMs use it to reach the Slow Slav. Not sure why. I always just thought that 3.e3/4.Nf3 and 3.Nf3/4.e3 transposed into one another.
In his Strategic Opening Repertoire, Watson recommended 3.Nc3 and 4.e3 as a way to reach the Meran. Why not go 3.e3 first as a way of avoiding the Slav Gambit (3.Nc3 dxc4) and the Winawer Counter Gambit (3.Nc3 e5)? Not sure.
Anyway, after 3.e3, it looks like any of 3..Nf6, 3..Bf5, 3..e6 or 3..g6 should transpose to regular lines after 4.Nf3. White has to be careful about trying to deviate with 4.Bd3 due to 4..e5! but he can try 4.cxd5 cxd5 5.Qb3 Qc7 6.Bb5 if he wants something different.
I replied to the middle part about 3. Nc3 earlier.
I think "Slow Slav" is not just anything with an early e2-e3, but this line in particular: 1.d4 d5 2.c4 c6 3.Nf3 Nf6 4.e3 Bg5 5.Nc3 e6 6.Nh4. There is also 6.Bd3, but that line has been considered equal from ancient times. Recent games don't change that assessment. You are correct that a lot of strong GMs are using 3.e3. For some reason I didn't notice that before. Now I spent some more time in the ChessBase tree, looking at how those strong GMs play it. I put the "main line" (if 3.e3 has such a thing) in bold. One thing to keep in mind after 3.e3 Bf5 is the related opening 1.d4 d5 2.Bf4 Nf6 3.e3 c5 4.c3 Nc6 5.Nd2 e6. If you think black is doing great against the London system then you will feel right at home in the 3.e3 Slav.
1.d4 d5 2.c4 c6 3.e3 Bf5 This is the important reply, avoiding transpositions.
(a) The big move is of course 3...Nf6, played 7x more often, when can happen 4.Nc3 e6 (4...a6) (4...g6), or 4.Nf3 Bf5 (4...Bg4) (4...e6) (4...a6) (4...g6).
(b) Another option for black is 3...e6. Black can follow up with the Stonewall, but usually just plays ...Ng8-f6, in which case I don't understand the move order.
4.Nc3 (a) 4.Nf3 e6 5.Nc3 see 4.Nc3.
(b) 4.Qb3 Qc7 5.cxd5 cxd5 see 4.cxd5.
(c) What the GMs are mostly playing is 4.cxd5 cxd5 5.Qb3 Qc7. This probably will transpose to the similar line with Ng1-f3 / ...Ng8-f6 in. If white wants to play this line, 3.e3 seems like a good move order. I have to say, I don't get the point of 6.Bb5+ at all, it must be one of those silly-looking computer moves based on a random tactic that humans, especially me, can easily overlook.
4...e6 Only move, although in the database 4...Nf6?! has been played 5x more often. That alone is a good reason to play 3.e3 against the small fry.
5.Nf3 The line 5.g4 Bg6 6.h4 is a specialty of Z. Kozul, and 6.Nge2 is another try. If black is worried about this then 3...Nf6 is the way to go.
5...Nd7 Again avoiding the transposition, although in the database 5...Nf6 has been played 20x more often, allowing the Slow Slav with 6.Nh4.
6.Bd3 (a) 6.Be2 h6 7.Bd3 has been tried, directed against ...Bf5-g6. 7...Bxd3 8.Qxd3 f5!? (ambitious, in practice black always plays 8...Ngf6, with a useful extra ...h6 in an equalish line) 9.cxd5 (9.g4!? works out okay for black 9...dxc4 10.Qxc4 Qf6 11.gxf5 Qxf5 12.Qe2 Ngf6 13.e4 Qh5 = Stockfish) 9...cxd5 10.g4!? g6 11.gxf5 gxf5 12.Bd2 Ngf6 = Stockfish. g2-g4 is not necessary for white, but is an attempt to punish black's extra ...h6 move. Otherwise, it will look very much like 6.Bd3. I suppose a disadvantage of ...f5 after ...h6 is black can no longer play ...Ng8-h6. An advantage is ...g7-g5 is supported.
(b) 6.Qb3 as I gave earlier is not especially good, e.g. 6...Qb6 7.c5 Qc7 8.Nh4 Bg4! as Jack Hughes correctly pointed out. White has done poorly from here, the most recent example being 9.h3 Bh5 10.g4 Be7 11.Ng2 Bg6 12.Nf4 b6 13.cxb6 Qxb6 14.Qd1 e5 15.dxe5 Nxe5 (1/2, 30) Akesson - Rosberg, Vasteras op 2017.
6...Bxd3 6...Bg6 is also a good move, when white can break with 7.e4, or with 7.Bxg6 hxg6 8.e4, or develop with 7.O-O Ngf6 8.b3 Bb4 9.Bb2, or with 7.O-O Ngf6 8.Qe2 Bb4 9.Bd2 a5.
7.Qxd3 f5 Both players should seriously look at 7...f5. It's not especially popular, but scores rather better than 7...Ngf6, which just transposes to a standard ...Bf5 Slav line, e.g. 8.O-O Bb4 9.Bd2 a5. Here black should also equalize sooner rather than later, so it seems black has a good choice on move seven.
7...f5 is rare enough, I don't see a main line. A couple of interesting examples:
(a) 8.O-O Bd6 9.b3 Ngf6 10.cxd5 cxd5 11.Nb5 Bb8 12.Ba3 Ne4 13.g3 a6 14.Nc3 g5 15.Nd2 h5 16.f3 Nxc3 17.Qxc3 Nf6 18.e4 dxe4 19.fxe4 Ba7 (1/2,78 on time?!) Cuenca Jimenez - Carlsen, Banter Blitz 2020
(b) 8.Bd2 Qf6! 9.Ne2 Bd6 10.Qb3 Rb8 11.Bb4 Bc7 12.Qa3 dxc4 13.Rc1 Ne7 14.Bd6 Bxd6 15.Qxd6 Nc8 16.Qc7 Ncb6 (0-1, 52) Miton - Hector, DEN-chT-1112 2011
(c) I think white should play 8.cxd5 to force 8...cxd5. Otherwise, Hector's 8...Qf6 will allow black to answer with the positionally desirable ...e6xd5. After 8...Qf6, black's further scheme is ...Bd6, ...Nh6, ...O-O, and then marching on the kingside. But 8.cxd5 cxd5 also looks quite solid for black, and black is doing fine there in the database.