SR-71 flew 2193 mph. How can this mathematically be Mach 3.3?
The SR-71 was only capable of Mach 3.3 flight at altitude. You can see the limitations in the pilot's operating handbook(POH) for the aircraft:



At higher altitudes the relative speed of sound is lower. If we use this handy NASA calculator in a standard atmosphere the relative speed of sound at 70,000 ft. is 660 MPH making your 2193 MPH equal to Mach 3.321



Its also important to note that the word "about" is often used as the SR-71 was flown to a maximum compressor inlet temperature(CIT) not a maximum Mach when doing a 3.3 run. According to the POH


  Mach 3.2 is the design Mach number. Mach 3.17 is the maximum scheduled
  cruise speed recommended for normal operations. However, when
  authorized by the Commander, Speeds up to Mach 3.3 may be flown if the
  limit CIT of 427°C is not exceeded.