Straight from Arlington: www.arlingtoncemetary.mil
"The 21-gun salute is not to be confused with the three-volley salute (or three-rifle volley) rendered at military honors funerals, which you might see or hear at Arlington National Cemetery."
American gun salutes and military funerals
A 3 volley salute is for funerals. Hence the Arlington Wreath Laying ceremony with the 3 Volleys. I dispute that anon who said it was 10 second intervals but other than that he is right. 5 seconds is standard between shots. So the videos claiming that the Wreath Laying Ceremony at Arlington is the Biden 21 gun salute is patently FALSE.
The burial service of a family member that I attended at Arlington was a rifle salute. I would have to dig out photos to see how many rifles there were — can’t recall if it was three or four. The video I saw of the Biden ceremony (I believe I saw it on AWK News - don’t know where he got it) was three cannon. And he counted 10 seconds between each fire. Two cannon alternating firing and one cannon in reserve in case of a misfire.