The average of first five multiples of 3 is:

First five multiples of three are:
3, 6, 9, 12, 15.
$$\eqalign{ & {\text{Average}} = \frac{{ {3 + 6 + 9 + 12 + 15} }}{5} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \frac{{45}}{5} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 9 \cr} $$

Alternatively,
Average = $$\frac{{ {3 + 15} }}{2} = 9$$