He could have shot 3 times, which still doesn't disprove another shooter outside or one outside with a silencer.
Well, of course ... and excluding *all* other evidence (which allows *any* conspiracy theory the "room to wiggle") darn near anything is possible. THAT'S where being a good detective/analyst counts for somthing; as much/as many pieces of the complete puzzle *have* to fit together, more or less seemlessly from beginning to end (THIS is where conspiracy theories usually fall apart - they work with a limted subset of information, and usually fall apart with the introduction of *other*, undisclosed information).
As Shooter 2.5 and others have pointed out - 'silencers' don't really 'silence', a 'report' (from the muzzle) would have still been heard by somebody (there were a LOT of people down there that day - and I suspect quite a number of plain-clothes officers and as well as SS agents in the crowd as well)...