Welcome to Communities and vSAN.
Issues with configure wizard are very often caused by vCenter being on a lower version than the ESXi (e.g. 6.7 GA vCenter 6.7 U1 ESXi) - please confirm you vC is on same or higher build.
Also, there is no benefit to specifying Fault Domains in a 3-node cluster - the nodes themselves act as Fault Domains by default in non-stretched clusters.
Could have sworn I was using the most updated version...I will need to get this looked up next working day.
Holy Molly, you're absolutely right.
Resolved the problem by updating to 6.7 U3B. Can't believe the solution was that simple, you live and learn I guess.
Many thanks, Bobkin !