The easiest way to think about this is to create Venn diagram. If the set of guns is entirely inside the set of firearms, and the set of knives has no members inside the set of firearms, then it follows that no gun can ever be a knife, whether or not there is or ever has been such a thing as a gun or a knife.
So (1) is a valid conclusion.
(3) does not follow, although your mileage may vary depending on the type of logic you are studying. This statement makes an existential claim -- that "some guns are." The issue is whether we can simply presuppose that there is some gun. Peter Abelard would have argued that the premise -- "All guns are firearms" -- is only true if a gun exists, and that therefore (3) follows. But modern logic handles this issue differently, holding that there is nothing in the premises to justify the claim that "there is some gun," let alone that some gun is not a knife.
(2) clearly does not follow.