if it's only on the 3rd cod, the sound is most likely the result of the cog being misaligned or having a tweaked tooth, or possibly some interaction with the chainline (doubtful) or derailer pulley when lined up with the 3rd cog (even less likely).
easiest way to start testing is switch the freewheel to another with the same speeds/spacing and see if that takes care of it. (I'm assuming this is a freewheel on this bike)