I'd guess and say that the foundation rotates to face the wind, because it seems like it would be easier for an active ballast system to work if each leg of the tripod was normally in roughly the same orientation to the wind.

In the picture, each of the turbines are oriented to face the other two legs of the tripod, but whether that is the design or it just happens to be illustrated with the wind coming in from that direction is not clear.

I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.