Ranma can eat for something like 10, and he's proud of that skill because a quick metabolism is required for quick recovery, so Ranma telling Akane she eats for 3 would be supposed to be a compliment, namely that she's a good enough Martial Artist, that she need 3 times as much food to fuel her training, as an average person does, which is of course still less than how much food Ranma can burn training.