There is evidence that there is a resonance with the frequency associated with G (happens on 2 strings when G is played). But still could be coincidence. So...
If there is a buzz on that instrument truely associated with that frequency, then we should be able to reproduce it in more places.
You say the 2nd string, 3rd fret (which is G) creates the buzz. So tune that string down a full step to D. Now G will be on that string on the fifth fret. See if the buzz is gone from the 3rd fret, but now has moved to the 5th fret. Now tune that string up two full steps to F#. In this case G has moved to the first fret. See if the buzz has followed, and only occurs now at the first fret.
If the buzz truely moves wherever G occurs, then I think that would confirm a resonance to that particular frequency on that instrument, and is not related to fret buzz, action, relief, etc.