I suppose, the variables x to z store the acceleration in m/s².
So, the total acceleration in m/s² of the device is sqrt (x*x + y*y + z*z).
The conclusion however, that the device fell to the ground is false. any acceleration can reach that value.
I have 2 ideas how to solve this particular Problem
1) acceleration has to be constant (within a range) for a given time (e.g the time of the fall from a hand to the ground which would be about 0.35 seconds.)
2) after the free fall, the acceleration goes insanely up, depending on the surface the device is landing on. Maybe you could catch that. :)