Yes, it's true that there were in fact 3 gear faults with SAS's Q400s.
However, only the first 2 were due to corroded threads in the retraction actuator piston.
With them, the rod end tore out of the piston. The gear fully extended, but collapsed after touchdown.
Of course, SAS senior management made lots of noise and did a lot of finger pointing.
When the third wouldn't extend it's gear and landed rather ungracefully, said management convened that same weekend and declared termination of all Q400 ops.
The accusatory din was deafening, until it came to light that a nose gear sequence valve was improperly modified for use in the main gear. An o-ring migrated into the wrong place, blocking an orifice, and hydraulically locking the gear most of the way up when they tried to extend for landing.
The Danish authorities have made this public.
The world wide fleet was inspected and IIRC only one non-SAS retraction actuator was corroded enough to disallow further flight.
So, what made SAS Commuter's Q400s so susceptible? Environment?
Wideroe and flybe operate in the same or similar conditions.
I would speculate that there was some combination of de-icing, washing, weather, etc, that conspired against them.
At any rate, the alloy steel pistons have been replaced with a new stainless alloy, which puts this issue to bed.
Stubs400