I think they have four guns in case one doesn’t work. I think the fourth is never used if nothing goes wrong with the others.
Recalling my Howitzer info from my BIL, blanks are part-filled with wax.
It makes a sticky mess on the inside and the soldiers grumble about cleaning them after salutes.
And a “ready” cannon is there in case one gets gummed up and can’t be loaded quickly enough for timing purposes.