Leap 15.3 has 2 more Repos:
Code:
18 | repo-sle-update                    | Update repository with updates from SUSE Linux Enterprise 15 | Ja        | (r ) Ja         | Ja             |   99      | rpm-md   | https://mirrorcache-eu.opensuse.org/update/leap/15.3/sle/
15 | repo-backports-update              | Update repository of openSUSE Backports                      | Ja        | (r ) Ja         | Ja             |   99      | rpm-md   | https://mirrorcache-eu.opensuse.org/update/leap/15.3/backports/
Add them first.