When two particles collide, the total momentum before the collision is equal to the total momentum after the collision, according to the law of conservation of momentum. Therefore, we can write:

$$mv = (m+2m) v_f$$

where $v_f$ is the final velocity of the combined particles.

Simplifying the above equation, we get:

$$v_f = \frac{mv}{3m} = \frac{v}{3}$$

So, the correct option is $$\frac{v}{3}$$.