When i purchased my V100 new.
The shop gave me 2x lock barrels with the bike.
So my question is. I have purchased a top box, and will be considering a pair of panniers very soon. Thats 3x lock barrels needed for said items, but only 2x lock barrels supplied.
It’s hard not fault your maths. ((O:
Is there a question for the team?
contact the manufacturer of the panniers and ask can they key you another lock
They obviously assumed no one would fit a topbox, only panniers. 
Aye, 2 barrels are for the panniers, topbox will come with it’s own key. Just the way it is I’m afraid.