the Big Three/Four/Five/etc.

A group of three, four, five, etc., people or things that are the most prominent, dominant, or established in an area, field, or discipline. These men came to be known as the Big Six of the civil rights movement, all of whom were instrumental in organizing the famous March on Washington in 1963. It's an open secret that members of the so-called Big Three corporate lobbies have the biggest impact on the crafting of trade legislation in the country.

the big Three, Four, etc.

the dominant group of three, four, etc. informal

1998 Sunday Telegraph The notion that someone outside the so-called ‘Big Four’—the ministerial group which meets before Cabinet —might be given such status is uplifting.