Any compressed air tank has a built in reg adjustable or not. Its impossible for any gun to operate on 3000psi. You need something to bring the pressure down for your gun to use. A preset tank is when the factory sets the regulator in the tank to release a certain psi all the time and is not adjustable. If a tank is preset to 800psi, than thats what comes out all the time. 800psi. An adjustable tank has a regulator where the pressure can be brought up or down to meet your guns needs. If your gun runs at 700psi to shoot and recock, you can adjust the regulator to let 700psi out of the tank. If your gun operates at 300 psi you can adjust the regulator in the tank down to 300. If you have a preset tank that puts out 700psi but your gun operates at 400psi. You'll need a 2nd regulator to bring the pressure down.
As long as you have a good inline reg (Palmer Stab and AKA Sidewinder come to mind) a preset will be fine. When you start getting into big time tourney ball and every fps counts I'd look into a more consistant tank, but with a Stab you should be around 3-4fps at very very most.
Only certain tanks will not creep as the pressure drops. The pressure is regulated by a piece of equipment called a regulator.
Say this is your setup
68 ci 4500 psi Preset Tank (insert brand name here)
Regulator (Ergo, gladiator take your pick this screw into an damn can't remember the name)
5 inches of steel hose connecting the asa to the Regulator.
You pull the trigger couple things happen and the Pressure of Gas inside the marker drops as air propels the paintball down the barrel. The regulator you have on you gun has a piston of some kind the force acting on this piston is now greater inside the steel hose leading to the regulator the piston opens and air flows in to fill the chamber. While this is happening the preset tank lets air into the hose as the pressure drops in hose do to the air flowing into the marker through the regulator. The pressure inside the marker reaches a point and the piston inside the regulator seals and prevents any more gas from moving into the valve chamber low pressure chamber or whatever prefer to use as big hole that holds regulated air thats ready to propel a paintball. The preset tank which are normally set around 800 seals itself as well do to the force acting upon the piston inside the preset tank. I'm sorry if its a little confusing I'm tired and wanted to give one more explanation.
Adjustable tanks themselves does not necessarily mean you can run in as a gas through the regulator has to have certain abilities before one should do this.