Where can I download UOX3 from?
You can download UOX3 from either the Downloads-page at UOX3.org or, if you want to download the source files and compile it yourself, from the UOX3DevTeam GitHub-page.
What exactly do I need to download?
Generally speaking, you should always download the latest available "All-In-One"-package. This might be a regular full release, or it might be an "experimental build" - which despite the name can more or less be considered intermediate releases between regular full releases, and have more in common with release-candidates in other projects than unstable/experimental versions. It is recommended to get the latest experimental build over the regular version, especially if it has been a while since the last regular full release.