At a constant pressure and number of moles of the gas the ratio T/V is equal to some constant. At another set of condition of temperature, the constant is still the same. Calculations are as follows:
  
 T1 / V1 = T2 / V2
  
 V2 = T2 x V1 / T1
  
 V2 = 100 x 3.0 / 300
  
 V2 = 1.0 L
Hope this answers the question. Have a nice day.