I have a 3000 psi tank and a new paintball gun that only can have 1000 psi's can I fill my tank up a third of the way and will my gun still work?

Your gun should run at a certain psi. it most likely has a regulater in the gun, now the tank is running at 300psi, when the gas or air enters the gun the regulater will adjust the gas or air down for what the gun needs to work. Just fill the tank all the way... TEAM LEGION