I'm not 100% sure what the problem you are trying to solve is all about, however, I guess fundamentally the question is how much weight do you have to stick on top of a piston with contents at 3000 atm to meet equilibrium.
Although your units are in old money (non si units), as far as I can tell, your calculations seem to make sense up until you start calculating P*A = mg. This as far as I can tell is unnecessary. You've worked out the pressure exerted as 44102.696 psi, which seems about right, although give some more consideration to the atmospheric pressure addition, I may be wrong, but I'm not sure that makes sense.
The area of the piston is Pi r2, which comes out as 0.022698 in2. Force required for equilibrium is Pressure X Area which will give you a figure in the ballpark of what you're expecting...