Let us consider two point charges q1 and q2, placed at a distance d.
Force of attraction,
F = k (q1q2)/d²________(1)
Now, if the force becomes F/3, then let the distance between the point charges be r
Now, force of attraction,
F' = F/3 = k (q1q2)/r²______(2)
dividing equation, (1) and (2),we get
3 = r²/d²
⇒r² = 3 d²
⇒r = d√3
If the distance between them is d√3, the force will be F/3.