Installing QGIS 3 on Ubuntu?

Just for the record, as of 02/24/2018, changing the repository from

deb http://qgis.org/ubuntugis-nightly xenial main

to

deb https://qgis.org/ubuntugis  xenial main

, and apt-get remove/install qgis worked. This installs qgis 3.0 and replaces 2.99dev.

The debian source: deb https://qgis.org/debian xenial main , however, does not seem to be update-to-date at present, and installs qgis-2.18.

Even if the website text is not updated yet, https://qgis.org/debian already contains 3.0:

As of June 2018, QGIS 3.2 installs on Ubuntu 16.04 with the following repositories:

deb http://qgis.org/ubuntugis xenial main

deb http://ppa.launchpad.net/ubuntugis/ubuntugis-unstable/ubuntu xenial main

ubuntugis-unstable is critical, as http://ppa.launchpad.net/ubuntugis/ppa/ubuntu xenial main does currently not contain a recent enough libgdal20 (it contains 2.1.3, but >= 2.2.0 is needed).