One of the early propeller planes (a Ford Tri-Motor) had three engines, one directly in the middle

Question:

One of the early propeller planes (a Ford Tri-Motor) had three engines, one directly in the middle and one on each wing. In this configuration, the plane can fly one with any two engines or the just the middle one alone. Assuming the reliability of each engine is .9, what is the reliability of the overall plane?