On February 23rd QGIS 3 was formally released. Since then the developers have been working on completing installers for Windows, Mac and Linux. As of yesterday the software is ready to be installed on every operating system! To install it you can visit the QGIS Download page.
QGIS always has two main versions available. There is the long-term release, which is the most stable because it is supported for one calendar year. In addition there is always a latest release. QGIS 3 falls into this latter category, it is the latest release. At the moment the long-term release is version 2.18. The latest release is still considered stable, but a new one comes out every 4 months.
WHICH VERSION TO CHOOSE?
If you want to have one version of QGIS without having to worry about updates, use the long-term release (2.18). If you want to experience all the new features that come with version 3 you will want the latest release (3.0). As mentioned last month, version 3 is a major new release with a lot of changes and new features.
On Windows machines you can have both. You can easily install both versions of QGIS side by side with no conflicts by using the OSGeo4W Network Installer. On Mac and Linux machines you can only have one version, so you need to choose.
QGIS 3 is fast, stable and feature rich. The main issue for many users is that not all Plugins have yet to be ported to QGIS 3. This is because this responsibility falls onto the individual plugin authors. With that said, at last count there were already 117 plugins available for version 3.