I find it helpful to describe what happens to the pieces of the system:
so at time 0s, the input switch bit changes from 0 to 1; as long as that input switch stays at 1:
at time 20s, the light comes on and the timer resets itself for another 20s.
at time 40s, the light stays one and timer resets itself for another 20s and the
at time 60s, the timer has timed to 20s three times, and the light goes out.
Is that how you understand the problem statement?
Do you know how to make a repeating timer i.e. timer that repeats itself?
Do you know how the various properties (.PRE, .ACC, .DN, .EN, .TT*) of a timer change as it is timing? Can you extrapolate that to the case of a repeating timer?
Do you know how to make a counter count to 3?
* Allen-Bradley notation; other OEMs have similar property sets.