During the announcement Apple will let us all know when it is coming out as well as when it can be ordered. Until then, you'll just have to wait like the rest of us.
The iPad 3 will be my first iPad just sold my Android Tablet to get the 3 when it comes out. So with the iPad 2 the only way to get on release date was to be at an Apple store or Best Buy? No getting it delivered on release day like the 4S? What are you thinking the release date will be after the March 7 announcment? Was the 2 the following Friday?
And I bet this time around Apple will do what they did with the iPhone 4S. Make a butt load of them and have them ready for everyone. With the 4S I was able to call around and most places had PLENTY.
From what I gather from industry reports, they have been manufacturing iPad 3's for a while now, so I guess (hope) it will be available shortly after it's announced.