qgis 3 - Installing QGIS3 on Ubuntu 16.04 Xenial: dependency problem

I am trying to install QGIS3 on my Ubuntu 16.04 Xenial.

Following the instructions on the QGIS Website I added the following lines to my sources.list:

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

I run

sudo apt-get update

and try to execute the installation:

sudo apt-get install qgis python-qgis qgis-plugin-grass

For each of the packages I get the answer that there are several missing packages:

python-qgis : Depends: python-qgis-common (= 1:3.0.0+24xenial-ubuntugis) but it is not going to be installed
           Depends: libqgispython3.0.0 but it is not going to be installed
           Depends: libqgis-analysis3.0.0 but it is not going to be installed
           Depends: libqgis-core3.0.0 but it is not going to be installed

           Depends: libqgis-gui3.0.0 but it is not going to be installed
           Depends: libqgis-server3.0.0 but it is not going to be installed
 qgis : Depends: gdal-abi-2-2-2 but it is not installable
    Depends: libgdal20 (>= 1.8.0) but it is not installable
    Depends: libqgis-analysis3.0.0 but it is not going to be installed
    Depends: libqgis-app3.0.0 but it is not going to be installed
    Depends: libqgis-core3.0.0 but it is not going to be installed
    Depends: libqgis-gui3.0.0 but it is not going to be installed
    Depends: qgis-providers (= 1:3.0.0+24xenial-ubuntugis) but it is not going to be installed
 qgis-plugin-grass : Depends: qgis-provider-grass (= 1:3.0.0+24xenial-ubuntugis) but it is not going to be installed

                 Depends: libgdal20 (>= 1.8.0) but it is not installable
                 Depends: libqgis-app3.0.0 but it is not going to be installed
                 Depends: libqgis-core3.0.0 but it is not going to be installed
                 Depends: libqgis-gui3.0.0 but it is not going to be installed
                 Depends: libqgisgrass7-3.0.0 but it is not going to be installed
                 Depends: grass740 but it is not installable

Most interesting is that some packages are not installable: gdal-abi-2-2-2, libgdal20, grass740

Any idea how to solve this?