As @ctgraham said in this thread
You can upgrade to 3.0 from any of the 2.4.x releases. But an upgrade from OJS 2.0.x, 2.1.x, 2.2.x, or 2.3.x may requires a different approach.
If you are in some of these versions the best choice is upgrade first to a 2.4.x version and then to 3.x.