In a nuclear fission reaction of an isotope of mass M , three...

Let the reaction be, $( P ) \to ( X ) + ( Y ) + ( Z )$

Let mass of each daughter nuclei be $m$, then mass defect can be written as $∆ M = M - 3m$.

Now the energy released will be in the form of kinetic energy of the daughter nuclei. Therefore, 

$\DeltaM{c}^{2} = \dfrac{1}{2}m{V}^{2} + \dfrac{1}{2}m{V}^{2} + \dfrac{1}{2}m{V}^{2} ⇒ V = \sqrt{\dfrac{2\DeltaM{c}^{2}}{3m}} = \sqrt{\dfrac{2\DeltaM{c}^{2}}{M - ∆ M}} \approx c\sqrt{\dfrac{2\DeltaM}{M}}$