A witness host is required for VxRail vSAN stretched clusters. It resides in Site 3 to act as a tiebreaker if a failure occurs at one of the other sites. The witness host can be a physical host, as used in this deployment example, or an ESXi host running inside a VM.
Configure the witness host as shown in the Witness host initial configuration section of the SmartFabric Services with Multisite vSAN Stretched Cluster Deployment Guide.