If the pressure acting on a given sample of an ideal gas at constant temperature is tripled, then

The question relates to Boyle's law which states that at constant temperature the volume of given mass of gas is inversely proportional to the pressure

V = K/p

PV = K

P₁V₁ = P₂V₂ where P₁ = P atm (pressure) P₂ = 3P

then P V₁ = 3P V₂

V₂ = V₁ / 3 so the volume is reduced to one-third its original volume.