I've seen pretty convincing studies showing opposite results:
http://www.econ.upf.edu/~iriberri/Personal_Web/Loreal_24_Feb...
TLDR: Using data from business competitions (L'oreal, undergrad and MBAs), they studied the performance of groups of three students, controlling for a lot of things. Groups of 3 women were by far the worst performers, while the best were 2 men and 1 women.
Why may that be? no clue..