Depends on ....
Who you are, what you are flying, who the regulatory authority is, and the approval the company has from that authority.
In a recent conversation with a friend who flies 737-300s in Europe, I was surprised to hear only one RVR is required for a CAT II or III for his operation. He said there was apparently a correlation between the ability to reach 60 knots in a specific zone during the rollout and the number of required RVRs. He also remarked it might not be the best way to do this type of operation.
The operation I'm with requires 3 for a CAT III, unless B or C is inop. Those can be a substitute for the each other.