When will it be announced?

Super Mario Galaxy was announced in May 2006.
Super Mario Galaxy 2 was announced in June 2009.

With these stats, it is possible that Super Mario Galaxy 3 would be revealed Summer 2012.  But the truth is that the Wii U will be released soon, so it will most likely alter this.  A couple guys did some calculations as to the Super Mario Galaxy 3 Release Date.  Super cool!  Check that out!

Super Mario Galaxy 3 video from Youtube