While we do not have official confirmation yet that Microsoft has started work on the third milestone of the upcoming Windows 8 operating system, we have the next best thing: Leaked information from Zukona, a user with a track record of reliable Windows build information.

Zukona posted the build string of one of the latest Windows 8 builds: 6.2.7956.0.winmain_win8m3.110228-1701

The win8m3 indicates that it is a Milestone 3 build, 110228 that it was compiled yesterday on February 28, 2011.
Microsoft Begins Work On Windows 8 Milestone 3 | Windows 8 News