3 gal harbor fright air compressor when i pull triger on nail gun the pressure dips down to low to use

If it shows the right tank pressure on the tank manometer (I believe 100 psi is the max for this compressor) and yet there isn't enough air for a single nail shot, there's definitely something wrong with the regulator or the nail gun.

If you're positive it's not the nail gun, Harbor Freight sells what looks like a suitable regulator replacement for something like $10.99. 

You should also be able to simply remove the original regulator and attach a new one outside of the compressor (but close to it) .

If you can't fix it, I guess it would be nice to be able to use it at least as a 3 gallon backup compressed air tank. You would still have to open the cover to remove the regulator which only works one way.