chaverim-
Try looking at all of them as limits. For numbers 1 and 3 the graph of the average will bounce lower and lower hitting bottom at 27.0 and 29.5 respectively, eventually as the number of calls whose duration you take into account approaches infinity the “bouncing” will be so small it will level off at 27.0 and 29.5 respectively.
The graph of the average of number 2 “telescopes” around 27 (think of a backwards telescope that extends forever- it goes higher and lower but the “mountains” and “valleys” get smaller and smaller) as you go along towards infinity. As the number of calls whose duration you take into account approaches infinity the “mountains” and “valleys” will level off at 27.0.