QGIS 3.0 is released. However, in the download page, there does not seem to be an entry (for Ubuntu (16.04) at least):
Latest Release 2.18.x Las Palmas (new LTR)
...
Long Term Release Repository 2.14.x Essen (previous LTR)
...
Development Version 2.99.x master [4]
https://qgis.org/debian-nightly
...
Which repository should be used to get the new QGIS 3.0?