There's the
Custom Reports Guide and the
Custom Statistics and Reports FAQ both of which can help you get started. The other reports in PT3 are essentially the other tabs; almost all of the PT3 data windows are reports that you could duplicate in the Reports tab.
There are also a bunch of reports you can download and import in the
repository which you can look at to see how they work.
One note: Beta 25 has a known issue with report filters not validating properly. This will be fixed in Beta 26 but would cause problems for creating new reports in Beta 25.