Yes you are quite correct, we discovered that we have broken the adminTemplate in v3 too. We have patched the issue in our latest code branch and will release a fix soon.
Bear in mind though that the adminTemplate should be considered largely deprecated now, we are encouraging customers to use the built in page editor / dashboard designer and menu editor. You should not create a copy of the adminTemplate unless you plan on creating a heavily customized app of your own (which requires you to maintain and update it each time we release a new version of mango).
We will also update the documentation to reflect this. Sorry for any confusion.
-Jared