I think it better to use for loop for databases. It is little bit cleaner.
hooks:
after_postgres:
- exec: |
for db in two_db thirdsite_db; do
sudo -u postgres createdb "$db" || true
sudo -u postgres psql "$db" -c "grant all privileges on database $db to discourse;"
sudo -u postgres psql "$db" -c "alter schema public owner to discourse;"
sudo -u postgres psql "$db" -c "create extension if not exists hstore;"
sudo -u postgres psql "$db" -c "create extension if not exists pg_trgm;"
done