you don't need to define the start() method inside your class, because it is just Runnable, not a Thread.
If you want to start your anonymous Runable, you need to pass it to the constructor of the thread, then start this thread.
In case if you want to use your Thread3 class, you need to provide additional constructor:
after that you can do in your main method something like this:
SCJP 6.0(95%), SCWCD 5(94%), SCJD (working on B&S v.2.3.1)