Infinity Ward plans to expand the player count playlist options in Gunfight in Call of Duty: Modern Warfare at some point.
Infinity Ward developer Joe Cecot tweeted out that they are actively working on bringing 3v3 and 1v1 support to Gunfight in the game.
The ability to have 1v1 and 3v3 matches for Gunfight exists in private matches, but is not available in the Public Playlist for Gunfight. That is restricted to just 2v2 option.
There is no ETA for when to expect this playlist changes to Modern Warfare – perhaps coming with Season 1 of the game or maybe at a later date.