I think most people do cut them down, to two per day but all mine at ages 4 years,3 years, and 19 months are still on 3 meals, and will likely stay that way now. I find smaller meals and more often suit mine, so they have breakfast, dinner and tea and a biscuit or two at bedtime to stop any nighttime hunger pangs.
Don't think there is anything written in stone that you have to feed only two meals, I guess most folk find either their dogs prefer it that way, or it is more convenient.