The game calculates in the counter-intuitive manner of 15+20+25 (60%). When you have Potential III, it tells you that you just have Potential III, and you are getting 25%, however, if you were to gain 100 XP from something, you would gain an additional 60 XP on top of it.
Contrary to FuzzyGold's math, what it means when they stack is that you add the percentile bonuses together before applying them. The way they are presented, you would think that they don't stack, but they do. I used to assume that they didn't stack until someone said that they did, so I went into the game and tested it, and sure enough, they do stack.
