The Big Three is one of the names given to the three largest strategy consulting firms by revenue: McKinsey, Boston Consulting Group (BCG), and Bain & Company. They are also referred to as MBB. The Big Four consists of the four largest accounting firms by revenue: PwC, Deloitte, EY, and KPMG.
The term Big 3 stems from the more common Big 4. Both are groups of global professional services firms, and over the years they have grown to compete for more work.
This article covers the two groups and explains the differences between them.