Architects Stanton Williams, Gehl Architects and Field Operations head up three of the teams, with landscape architects Martha Schwartz Partners, Gustafson Porter and Gross.Max leading the remaining three.
The international competition received 101 expressions of interest and the shortlist announcement was initially due in the summer of 2008.
This was delayed until early November but the teams only received word yesterday that they were in the final. No date has yet been confirmed for their second-stage submission.
The square is scheduled for completion in 2013 at a cost of £6 million and will occupy the present site of King's Cross Station's Euston Road concourse. Work will begin when the concourse is demolished following the completion of the John McAslan + Partners-designed replacement.
The new public space will take advantage of the Grade I-listed south facade of the station. Designed by Lewis Cubitt in 1851, it has not been seen properly for over 30 years.