Install QGIS 3.x on Ubuntu 18.04

The ubuntugis ppa holds only LTR versions, currently QGIS 2.18.28, see https://launchpad.net/~ubuntugis/+archive/ubuntu/ubuntugis-unstable/?field.series_filter=bionic.

If you want QGIS 3.4, you can take the https://qgis.org/ubuntu/ repository without any ubuntugis dependencies. You might have to remove any packages you have from ubuntugis before installing.

https://qgis.org/ubuntugis/ has support for bionic as well, but faced some issues recently. The QGIS packages are built against GRASS 7.4.3, but the ubuntugis ppa has meanwhile switched to GRASS 7.6.0. So I don't suggest that at the moment.

The situation is different for Xenial, because the main ubuntu repo for that does not include GDAL 2, which is mandatory for QGIS 3.

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UPDATE

Ubuntugis unstable now has QGIS 3.4.14 for bionic.

I had the same problem. What worked for me was mainly following this article, specifically:

1.Open /etc/apt/sources.list with

sudo nano /etc/apt/sources.list

2.If there is any mention of QGIS, delete that line. Then, copy there this line:

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

3.Then, run this

sudo apt update && sudo apt-get install qgis python3-qgis qgis-plugin-grass

4.If it didn't work, go back to step 1 but instead of deb https://qgis.org/ubuntugis bionic main, try one of the following:

deb https://qgis.org/debian bionic main
deb https://qgis.org/ubuntu bionic main