Don't under-estimate Airbus engineers, 3 sensors, median filters, time delays are all included to minimize false alarms, but what is left is the remote possibility that any 2 may fail in exactly the same way at the same time - so check out the following, especially the 'foreign filing date Sep 23, 2009' :-
Quote:
United States Patent Application 20110071710 Kind Code A1 Puig; Stephane ; et al. March 24, 2011
METHOD AND DEVICE FOR DETECTING AN ERRONEOUS SPEED GENERATED BY AN AIR DATA INERTIAL REFERENCE SYSTEM
Well I agree that from Airbus engineers point of view double or triple Pitot's failure is a very low probability event. My university used to deal with "aircrafts" going out of the atmosphere and then back in, so Pitot's were one of the many sensors allowing computer to integrate positions and speed. And cross checking was a must. Remember this knowlege was around from 12 April 1961 and culminated by Russian space shuttle flown 100% by on-board computers.
Very sad that we paid such big price for engineering mistake or in fact "overseeing" the probability of failure being not so low. For example bird strike coud give the same result for all three tubes, since birds do fly in formations (flocks).