Two points A(X1, Y1, Z1) and B(X2, Y2, Z2)
The distance |AB|=√[(X1-X2)^2 + (Y1-Y2)^2 + (Z1-Z2)^2]
For example, A(1, 2, 3), B(2, 3, 4), Then|AB|=√[(1-2)^2+(2-3)^2+(3-4)^2] = √3 = 1.7320508075688772.