Hi eckhard
If one assumes that the ASI reading is actually IAS, then entering the three values into a computer to calculate the TAS would yield three different results. This cannot be correct, as the three aircraft are flying at the same TAS.
Indeed. But the TAS derived would not be correct in each case because the instrument error is different in each case. Nonetheless the reading on the dial is still by defintion the IAS.
If you wanted the precise TAS you would need to know the actual instrument and position error of your particular installation but you don't have that. Instead you have the AFM IAS/CAS correction (if you even bother) and you assume it is good enough because it has been certified within a tolerance.
You could equally have all 3 gauges reading the same IAS and all 3 aircraft maintaining formation. But after checking the TAT in each aircraft and "entering the three values into a computer to calculate the TAS", you would again get 3 different values because all the TAT gauges are not quite reading the same. And again it cannot be correct because the aircraft are actually "flying at the same TAS". Or for TAT substitue altimeter reading and same problem again. Welcome to the real world!