Microburst2002, it all depends on the OPS specs and country regs you're flying to. Under EU OPS one can commence the approach regardless of reported RVR and may not continue beyond ABP if it falls below the required one. For CAT IIIb you only require single RVR anywhere along the RWY of 75 m, that's all. Once you passed ABP RVR is of no relevance any more it's a simple reliance on sophisticated automation therefore the AH. It's a different ball game with FAA folks once again their specs stipulate all three RVR to be above and available to commence CAT III b approach however once passed the ABP the approach can be continued regardless of RVR as under EU OPS as well. Approach ban policies vary with countries thus make sure to check the applicable regs.
