How to install QGIS-3 (2.99) on Ubuntu 17.10?

This is what worked for me for qgis3 (2.99):

Add following to /etc/apt/sources.list and remove previous qgis entries

deb http://qgis.org/debian-nightly artful main 
deb-src http://qgis.org/debian-nightly artful main

Then, in command-line:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys CAEB3DC3BDF7FB45
sudo apt-get update
sudo apt-get install qgis python-qgis

I had to use debian-nightly and also apt-key to get it work.

Ubuntugis is not provided for higher than xenial and is not needed for Qgis 2.99 unless you have xenial or older. There may be an update to ubuntugis when the next ltr of Ubuntu comes out (18.04)

http://qgis.org/ubuntugis-nightly/dists/

Just use the regular debian repository to install qgis.