An object moves with speed $$v_1, v_2$$ and $$v_3$$ along a line segment $$AB, BC$$ and $$CD$$ respectively as shown in figure. Where $$AB = BC$$ and $$AD = 3AB$$, then average speed of the object will be 

A

$$\frac{v_1 + v_2 + v_3}{3}$$

B

$$\frac{v_1 v_2 v_3}{3v_1 v_2 + v_2 v_3 + v_3 v_1}$$

C

$$\frac{3v_1 v_2 v_3}{v_1 v_2 + v_2 v_3 + v_3 v_1}$$

D

$$\frac{v_1 + v_2 + v_3}{3v_1 v_2 v_3}$$