if the mass of the body is increased three times,how much velocity should be reduced so that the kinetic enery reamins the same ?

Let the initial velocity be u and new velocity be v

KE = 1/2 m u²

New KE = 1/2 (3m) v²

Now

1/2mu²= 3/2mv² 

v = u / √3

So, v / u = 1/ √3

Vlocity should be reduced to 1/√3 times of initial value.