In engineering, redundancy is the duplication of critical components
or functions of a system with the intention of increasing reliability
of the system, usually in the form of a backup or fail-safe.
In many safety-critical systems, such as fly-by-wire and hydraulic
systems in aircraft, some parts of the control system may be
triplicated, which is formally termed triple modular redundancy (TMR).
In reliability engineering, dual modular redundancy (DMR) is when components of a system are duplicated, providing redundancy in case one should fail.
I think your confusion lies within the formal terms. If you think of a redudant system as a DMR system the term TMR makes more sense and takes away your confusion about the sensor.
So if you add redundancy to the system (there is no such thing "single" redundancy) you can choose a DMR system or a TMR system. Usually people would think of a DMR system unless specified otherwise. So to keep consistent with your terms, what you would define as single redundant is the same as a DMR system. And then, if you have 3 modules in total, you speak of a TMR system.
It all refers to the total number of modules, not the number of added modules.