CloudNativePG operator should be installed in namespace "{{ .Release.Namespace }}".
You can now create a PostgreSQL cluster with 3 nodes in the current namespace as follows:

cat <<EOF | kubectl apply -f -
# Example of PostgreSQL cluster
apiVersion: postgresql.cnpg.io/v1
kind: Cluster
metadata:
  name: cluster-example
spec:
  instances: 3
  storage:
    size: 1Gi
EOF

kubectl get cluster