What are three measurements that are helpful when working with a gas?

Temperature, Pressure, # of moles, volume of container.

If you know any three of those you can find the remaning by equation PV=nRT, where P=pressure in atmospheric units, V=volume in liters, n=number of moles of the gas, R=universal gas constant (which, in this formula, is equivalent to .08206), and T=temperature in Kelvins (not centigrade, but kelvins).