Not sure what kind of oatmeal you are talking about. 40 grams of rolled oats is going to give you 150 Calories of which only 108 Calories will be from carbohydrates that will get converted to energy faster than the other stuff. 40 grams of rolled oats is also the serving size that I eat for breakfast sometimes and that will fill me up till lunch if I'm not doing much else.
150 Calories of carbohydrate being more or less what I put in one bottle for 50 minutes of riding, I'd imagine by 3 hours of riding this thread was originally about you'll have a pretty full stomach with more than 3 servings of oats in you, and probably coming out every orifice since oatmeal does give one a lot of soluble fiber to improve bowel movement. And exercise on a full stomach makes me want to puke.
I suppose you could just do one serving. But still one bottle of anything isn't going to be enough for 3 hours. You'll need to make more allowances for hydration whether you put anything in your bottles or not.