Btw, one cannot compare these three and I would recommend using all three if your citation count is less than 100, which is the case for most of us.
Scopus updates faster than ISI Web of Science because it is part of Elsevier. Google scholar picks up a lot more citations that appears online (such as books, magazine articles, theses, etc.) and not so much that are indexed in the traditional way. Web of science is accurate but takes a long time to appear in the citation search if you were to do one. It is behind Scopus by 3-4 months, in my opinion. So, use all three and see for yourself.