The Panels Demo is a good way to get some experience with Panels 3. It is featured on the project page but never made it to the documentation. Go to http://panels.demo.logrus.com/. Login and password were demo, but unfortunately, both currently and for a long time (see comments at the bottom of this page) the login has not worked.
Some things to notice that may help others prior to getting some documentation for panels 3:
1) Variants- when you create a panel, that is the first variant by default. See http://www.angrydonuts.com/panels-3-0-and-ctools-1-0-official-released for other uses of variants
2) Your blocks and panels can conflict with each other. You can disable the blocks from the panel page- See http://drupal.org/node/569928
3) Views plays really well with Panels. Note on the Panel Demo site the only view that is enabled when you go to add content to a panel is featured_stories. Try enabling another View like Archive view and then go back to the pane and add content with a view. You will see archives in the views selection
4) Read the "About" section in the Panels Demo for a description of demo site.