This is a community plugin and is thus not built-in to proto. In order to use it, first either add it to your global or project-based .prototools
by running:
proto plugin add k3d "source:https://raw.githubusercontent.com/appthrust/proto-toml-plugins/main/k3d/plugin.toml" --global
proto install k3d
proto plugin add k3d "source:https://raw.githubusercontent.com/appthrust/proto-toml-plugins/main/k3d/plugin.toml"
proto pin k3d latest --resolve