Chemistry - Three stacked arrows, what does it signify?
There is an explicit answer in section 3. Other Horizontal Arrows of prof. Santiago Alvarez' published work [1, p. 594]:
Finally, multiple arrows (7) are used as an ellipsis for several intermediate steps in a chemical reaction.
- Alvarez, S. Chemistry: A Panoply of Arrows. Angew. Chem. Int. Ed. 2012, 51 (3), 590–600. DOI: 10.1002/anie.201101767.
It means that after multiple (unspecified) steps, the substrate on the left is transformed into the product on the right. It can be mechanistic steps or synthesis steps (to infer from the context).
Here is an example from the literature where the authors have added "steps" to make it perfectly clear. (Nicolaou, Kyriacos Costa, et al. "The Diels–Alder reaction in total synthesis." Angewandte Chemie International Edition 41.10 (2002): 1668-1698.)