There's a tradition at Wikipedia of predicting when a particular milestone will be reached. Earlier expectations about Wikipedia were laughably conservative. While people have been guessing topics (rather random), there's sadly no page for the three millionth article. Given that since May the English Wikipedia has been increasing at a rate of \~1,300 a day, I'm expecting Wikipedia will hit this milestone in one week!
John Broughton on 2009-08-11
There is a pool (closed August 2008) for when the 5 millionth article will be created, but such no pool the 3 millionth or 4 millionth article. There is, however, a pool regarding the topic of the three millionth article.