They're grouped together because this trio have noticeably higher average revenues than their nearest competitors.
In addition, they all focus on strategy consulting at board level, primarily for Fortune 500 and FTSE 250 companies.
Who makes up the Big Three?
The Big Three is made up of Bain & Company, the Boston Consulting Group, and McKinsey & Company.
Why work for them?
The Big Three invest heavily in their greatest asset – that is, their people. As a new joiner, you'll receive fantastic training in basic business concepts, technical skills such as Excel or Powerpoint, and soft skills such as presentation techniques or building relationships with clients.
A second motivation for joining a Big Three employer is getting to work on high-profile cases and gaining exposed to the upper echelons of some of the world's most famous organisations.
Note, however, that these three firms have incredibly select recruitment practices, hiring mostly top-scoring graduates and MBAs from top universities. In recent years, their intake has diversified a little, with former lawyers, NASA astronauts and even opera singers joining their ranks.
Bain & Company
UK headquarters 40 Strand, London
Global revenue 2016 $2.3 billion
Number of employees 7,000
Number of alumni 13,000
Notable alumni Meg Whitman (CEO of Hewlett-Packard), Mitt Romney (former presidential candidate), Mark Pincus (founder and CEO of Zynga)
The young upstart of the Big Three firms, Bain & Company was established by ten Boston Consulting Group employees in 1973 who wanted to test out a more pragmatic approach to business than that practiced at BCG.
The experiment was successful and the company became most notable for winning new contracts on the basis of quantitative performance. A favourite practice of founder Bill Bain was to show prospective customers the increase in the price of current clients' shares relative to the Dow Jones Industrial Average. An increase that he claimed Bain had generated.
Today Bain is known for its continued focus on results and close relationship with Bain Capital, an investment firm specialising in private equity – many employees exiting Bain & Company join its ranks.
The "Bainies" are reported to be a bit more fun than other Big Three employees, with the firm cultivating a "work hard, play hard" culture.
The Boston Consulting Group
UK headquarters 20 Manchester Square, London
Global revenue 2016 $5.6 billion
Number of employees 14,000
Number of alumni 20,000
Notable alumni Clayton Christensen (Harvard Business School professor), Jeff Immelt (CEO of General Electric), Indra Nooyi (CEO of PepsiCo)
The Boston Consulting Group (aka BCG) is known as the academic powerhouse of the Big Three. It was credited by Harvard Business Review in 2012 with coming up with two of five concepts (the Growth Share Matrix and the Experience Curve) that changed the business world.
The firm has since continued to regularly introduce new concepts and highlight trends drawn from its in-depth research.
BCG has also been consistently ranked by Fortune magazine as one of the "Best Companies to Work For" – it offers more generous benefits than the other two Big Three members and places a greater emphasis on work-life balance, promoted by initiatives such as "predicted time off".
BCG people are often characterised in the industry as especially brainy, known for spouting academic theory but also for using complex analytical methods well to find good solutions.
McKinsey & Company
UK headquarters 1 Jermyn Street, London
Global revenue 2016 $8.8 billion
Number of employees 14,000
Number of alumni 30,000
Notable alumni William Hague (UK Foreign Secretary), Sheryl Sandberg (COO of Facebook), Chelsea Clinton
McKinsey & Company is the biggest, oldest and most prestigious of the three firms, with one of the most powerful alumni networks in the world. The company has been described as “the CEO factory”, with the odds of a “McKinseyite” getting to run a publically-listed company estimated at 1 in 690 – the shortest of any employer in the world.
The company remains at the forefront of innovation. As competitors attempted to eat into its profits, it created McKinsey Solutions, technology-based analytical tools that can be embedded into a client's own systems, providing ongoing analysis without the need to utilise staff or employ external advisers.
With a reputation and results like theirs, they've got good reason to boast about what they're doing.
Although the Big Three are the world's biggest consulting firms with experience of working with most types of business, those with a specific industry interest might have a more satisfying working life at a boutique consulting firm.
For example, if you're aiming to specialise in financial services you're likely to encounter a more diverse range of cases at Oliver Wyman. Alternatively, if your interest lies in e-commerce, then Javelin may be the right place for you.
It's also worth noting that smaller consulting firms, such as Value Partners or Parthenon, could provide you with more interaction with clients, as case teams are smaller, and quicker promotion to management positions.