On the contrary, a 2-sensors systems, need at least one mechanical tie to be stabilized.
In the 2-sensors systems, the presence of one or more mechanical ties alters the measurements of vibrations, as the vibrations may not completely go through the sensors but, at least partially, they go through the mechanical ties.
[When mechanical ties are present, a "parametric" modeling of the measuring system is necessary, as the system changes every time that the tie changes (i.e.: micro variations from one unit to another, variations of temperature, replacement of parts of the system). Therefore, a simple self-calibration may not be enough, but a "factory calibration" is needed, to compensate errors in measurements.]
In 3-sensors VQI System, the centrifugal force caused by the unbalance goes completely through the sensors as there is no other mechanical tie. In such way, the vibration force is therefore completely known and measured. This makes sure that in every condition and with any type of wheel, wide small or, measurements will be always perfect.