The volume of a given amount of gas is related to the temperature and pressure by the Ideal Gas Law by P*V=n*R*T
or V = k*T/P
The volume is directly proportional to the temperature and inversely proportional to the pressure.
If the volume is tripled, the volume decreases to one third the original volume. And if the temperature is doubled, the volume increases to twice the original volume. If both the changes take place simultaneously, the volume would become 2/3 the original volume.
If V' is the changed volume and V is the original volume we have V' = k*2*T/3*P = (2/3)*(k*T/P) = (2/3)*V