As white, I've been seeing a lot more of the Triangle stuff lately, probably the past 3-6 months or so (anyone else seeing this more too?). If it gets played from the Slav order, 1. d4 d5 2. c4 c6 3. Nf3 e6, I play 4. Qc2 to avoid an early e3. But what do y'all play if it arises from the QGD move order, 1. d4 d5 2. c4 e6 3. Nc3 e6? If 4. Nf3, then we could get into the Noteboom, which I am not really wanting to study. But if 4. e3, black can switch to a Stonewall with 4...f5 which is annoying to play against. Is it worth learning the 4. e4 dxe4 5. Nxe4 Bb4+ 6. Bd2 Qxd4 7. Bxb4?
4.e4 is the marshall gambit and its very aggressive,
black will have to defend for some time,
the gambit is also sound not a speculative
gamble that black will go wrong