I had always assumed they slid on the truck hence you could swap them in and out and explained why they provided three with the truck. Bad assumption?
I wanted to chime in, since I owned a couple of the loose stickers for a short time.
I cannot be certain, but I would seriously doubt that the design was that you could change out the stickers. I'd figure, if the intent was to change them out, that they'd have created little truck-side-cards, that would slide onto some holding bracket, rather than a thin, peel-away sticker. This is also what you might call a "cheap" toy, and as such, the sticker-to-be-stuck option makes more sense, and is cheaper and easier to produce. Because of that, I think the odds are that the stickers were meant to be stuck on the truck, and aren't held on by any other means.