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.

Reference

Alvarez, S. Chemistry: A Panoply of Arrows. Angew. Chem. Int. Ed. 2012, 51 (3), 590–600. DOI: 10.1002/anie.201101767.