Why are there 3 rounds placed in the flag at military furnals?

To represent the 3-gun salute. Each firer shoots 3 rounds in volley. If there are 5 honor guards, then it appears that there is a 15-gun salute, but each fired weapon doesn't count as 1. Many confuse a 21-gun salute with having 7 or even 21 guns, but in reality there is a battery of field artillery pieces.