In Helm v3, the release name is now mandatory as part of the commmand, see helm install --help:
Usage:
helm install [NAME] [CHART] [flags]
Your command should be:
helm install my-nginx stable/nginx-ingress
Furthermore, Helm will not auto-generate names for releases anymore. If you want the “old behavior”, you can use the --generate-name flag. e.g:
helm install --generate-name stable/nginx-ingress
The v3 docs are available at https://v3.helm.sh/docs/, but as it is a beta version, the docs will not be accurate for a while. It’s better to rely on the CLI --help, that is auto-generated by Go/Cobra.