If you sustitue the TD for the mid point and the midpoint is less than 200 (our minima) you would go around outside the marker. If you just consult the table and assess that one of the three is inop then you would continue.
Hardly! You would need the midpoint RVR to be at or above your required touch down minima if TD was inop. You substitute the missing RVR intelligently. You don't simply say 2 out of 3 are working (but both below my TD minima) so continue!
The simple idea is that the approach should have a reasonable chance of success for a safe landing. Your logic of using the 2 reported RVR (both less than your TD minima) has little chance of success.
Have a read of
http://aviation-safety.net/database/...?id=19651027-0 . We learned from accidents like that and introduced approach ban minima. Don't try to circumvent the rules - they are there for a reason.