Mean is sum of data divided by how many; median is central value when data placed in order, but in the case of an even number of data, it's the average of the central pair.
......MEAN....................MEDIAN
(1) (5+3+6)/3=14/3=4.7....5
(2) 14/4=3.5............(3+4)/2=7/2=3.5
(3) 11/4=3.75..........(2+3)/2=5/2=2.5
(4) 0.................................-1