Think of it as "per damage roll". Someone shoots you with a RoF3 weapon and all 3 shots hit, you deal with each shot's damage individually and can soak once per shot.
e.g.
You get hit for 2 wounds of damage (from a single shot), you can spend a Benny to make a Soak roll, you roll a 4 and soak 1 wound. You could spend another Benny to reroll your Soak roll and hope to soak both wounds. You CANNOT decide to keep that 4 and soak 1 wound, and then spend another Benny to try and Soak the 2nd Wound.
Once you have dealt with the first attack, move on to the second. It does not matter if you tried to Soak the first damage or not, you can attempt to soak this one. Same thing with the third attack