OpenShift 4.5 introduced support for "worker-less" / compact / 3-node clusters. The current documentation does not seem to document this anywhere. My understanding is that this installtion method is only supported for non-provider-integrated clusters, which the documentation refers to as "bare metal". The current metal installation documentation specifies that 2 workers are required, and does not detail how to perform a worker-less installation or even that it's supported: https://docs.openshift.com/container-platform/4.5/installing/installing_bare_metal/installing-bare-metal.html
This information was added after Erik filed the bug, and he's confirmed that the live version is acceptable: https://docs.openshift.com/container-platform/4.5/installing/installing_bare_metal/installing-bare-metal.html#installation-three-node-cluster_installing-bare-metal