could be a few things. one, your wastegate is not opening properlery. two, the popoff valve could be bad. start there if its neither, we will have to go in deeper to figure out the problem.
Actually Brian, you got that backwards, the waste gate is not closing or opening to early. Disconnect the vacuum hose to the wases gate and plug it. Then give the car a run and see what the boost is. Also make your stock gauge is reading the boost correctly if you are using it. Brian was correct in saying the pop off valve could be bad. Remove the valve and plug it and run the car and see what the boost is. Dont worry to much about the boost as the stock turbo will not get you that much boost to damage the engine, just test the parts that way and determine if it is the turbo or not.
Assuming you really only have three psi boost, do the things mentioned above and if that all checks out good, check for a restricted exhaust. Old and damaged/plugged cat converters will kill the boost fast. Open up the exhaust and see what happens.