How to set up repository metadata signing for deb content in Pulp 3 is described in this thread: Pulp_deb for pulp3 in Katello
Note that the process has changed somewhat with the pulp_deb 2.10.0 release since the SigningService model in pulpcore was changed. If you use the scripts from the pulp_deb repo, make sure you use the right version for your installed version of pulp_deb.