Persons 1,2,3 are voting for candidates A,B,C with the following individual preference lists:
Person1: A>B>C
Person2: B>C>A
Person3: C>A>B
Using a majority voting rule, we can see that A would beat B, since A>B in two out of three peoples' preference lists. Similarly, B>C and C>A.
But then A>B>C>A which is obviously unreasonable.