I thought it was rifle fire. I read “canon” in your post and went with that, without thinking it through. My understanding was three shots for a funeral, versus the four shots for a POTUS. I can’t recall the source.
It is rifles. Called 3 volleys, 21 total shots. 'Guns' are artillery guns, never to be mistaken for 'rifles', except for 'naval rifles' but there is always a ship with them.