The article: Martín-Martín, A., Orduna-Malea, E., Thelwall, M., & López-Cózar, E. D. (2018, August 15). Google Scholar, Web of Science, and Scopus: a systematic comparison of citations in 252 subject categories. https://doi.org/10.31235/osf.io/42nkm
was summarised by its authors in the LSE impact blog on December 3rd at
The scholarly article ends by saying "In conclusion, the inclusive paradigm of document indexing popularised by Google Scholar facilitates discovery of not only the most well-known sources, but also of sectors of scholarly communication that were previously hidden from view." but it highlights the tradeoffs, as Google Scolar has issues of its own. Essentially I think the message is what you may well already know i.e. if you want a thourough search, search all 3 of them.