psql, I try to connect to a database and I get "psql: FATAL: Peer authentication failed for user ", why?
You're not asked for a password because the pg_hba.conf configuration file in CentOS says that a connection through a Unix domain socket implies the peer authentication. peer succeeds only if your OS user is the same as your database user. It's a reasonable default.

You may edit pg_hba.conf to indicate that anyone can connect locally to davide3 to certain databases or all databases, or more generally whatever policy is best suitable in your case.

The file is documented at:
https://www.postgresql.org/docs/current/static/auth-pg-hba-conf.html

Alternatively, keep the default policy and try to add -h localhost to psql invocation. This should trigger a different authentication rule (md5) in the default pg_hba.conf that will ask for the password.