How is it that on three different occations something has disabled both the primary AND the alternate gear extension?
It didn't. The gear extended properly on both the 1st and 2nd accident but didn't lock down on the starboard side. On the 3rd occasion there was something there that shouldn't have been (the O-ring), causing the system to malfunction and the starboard gear failed to deploy.
The most reasonable cause at the moment looks like a badly designed actuator on early Q400 aircraft causing accident's #1 and #2. With accident #3 caused by poor maintenance procedures and/or fitting of the new actuators.
SAS should have waited on the outcome of the 3rd accident before making their decision on the Q-400 which in hindsight looks like a mistake. Flybe's response was certainly well thought out and proper.