Mid Point in three dimensions

Highcharts Example
Definition:The point of a line segment or curvilinear arc that divides it into two parts of the same length.
The Midpoint M of the line segment joining the points (x1, y1, z1) and (x 2, y 2, z2) is

Example :
Calculate the Midpoint of the line segment joining the points (3,4,5) and (6,7,8) is,

X1 = , Y1 = , Z1 =
X2 = , Y2 = , Z2 =
 
Midpoint =