Weird thing is that I rarely seem to sit opposite three men or three women in a row. Certainly not on the way into work. Coming home, I'm more likely to get the 'lads' or 'girls' night out there's something about a morning commute which means sexes are "shuffled" a bit more. Particularly when random strangers are concerned.
But perhaps it's just me. It's difficult to "prove" my theory, but I had a look at the Tube Eye Pool on flickr - which is the closest thing to vaguely regular photographic view of commuters I could think of. Of the last twenty pictures which had three or more people sitting in a row (with no spaces) only seven were "three of a kind". There were only two sets of three women amongst them, so even with my rubbish maths, that's fairly low odds of getting "three of a kind" made up of women.
Try it on the way into work and, if you're lucky enough to get a seat, - see if you a) actually get to sit opposite a three in a row, and b) can correctly predict the sex of the next three in a row you get. If there are a lot three women in a row, my random control group above is completely bogus.