With 3 PSI you are able to push the beer up about 83 inches over the level in the keg. That is enough to push it through the handpump and out (unless the keg is in the basement and the handpump upstairs).
I have a low pressure LP gas regulator on the keg gas in, as a poor man's breather. It pushes the beer up about 10 inches, so with the pump on the bar, it is OK. They talk of this set up in the new BYO. You have to remember that a cask if vented to atmospheric pressure with the soft spile.
If you want to maintain pressure after serving, disconnect the low pressure, and put the CO2 on at the pressure you desire for the beer.