I know this is a really old thread, but just stumbled across it when I was looking for something else.
Bit of info that might be helpful......... Even if you did try to use 3, you wouldn't want to as you could get to the two on the rear easy enough, but it would be dangerous to get to the middle one on the front to put the safety stands in place which are critical if you want to do anything under the car as if the line blows it'll drop straight on you pretty easily. If you use 4 it's easier to just reach in from the side and put the safety stands in.
Second thing is regarding pressure. Obviously, most of the time you would use an external supply with either air or nitrogen as others have said, but we chose to have the ability to do both as we can plumb in external supply, but most of the time I use the on board supply which is a small dive bottle (3000psi) regulated down to 325psi. Ideally I would prefer something like 400psi, but the electronic manifolds / valves that I use don't like working over 325psi so it still works, just goes up a bit slower...... but only like 10 seconds instead of say 3-4 seconds with higher pressure.
Basic science says the size of the airjacks and the weight being lifted will tell you what pressure you need, so there's no point saying that you'll only need 325psi to lift your car etc etc since yours will probably be heavier than ours for instance and also your airjacks might be smaller diameter or larger diameter so the pressure needed will be different, but I would suggest that you would usually need at least 300psi at worst. I was told initially I would be able to use 200psi etc so I never bothered to check the numbers, but the reality is it was not even close.......
I get approx 7 lifts from a small bottle, so I have two and just swap them over which is pretty quick and easy.
I'm not allowed to run the bottle for sanctioned motorsport events, but for normal track days it's fine so makes life super easy to work on.