What do you mean by keystone client? Keystone client has 2 parts, a library and a shell which uses the library. The shell is the command line client.

Keystone client library supports v3. So if you are writing a python code, you can call v3 api using the client library. Shell ( command line client ) on the other hand doesn't support v3.

There is an initiative to move all command line clients to openstack client project. Most probably you will be getting v3 support for command line clients from openstack client and not from keystone client. Going forward keystone client will be pure library. This will be true for all the openstack services. Check https://github.com/openstack/python-o... for openstack client

There are some openstack services which supports v3 token api from command line without waiting for openstack client. e.g cinder. If you download cinder client, you can call keystone v3 token api.