Preschoolers have such a flair for the dramatic! My guess is that two things are going on here. The first is simply a matter of verbal and cognitive development. As an adult, you have a much deeper understanding of the subtleties of language. Your 3-year-old sees the world in much more black-and-white terms.
That's why a young child will state that he hates spinach, all the while making a face and sticking out his tongue in disgust. An adult with the same aversion will simply state that she dislikes spinach, and do so without the attendant melodrama. The cure for that part of the problem is patience. It will go away as your child matures.
The second issue is that your son has learned that you react when he says he hates something. It's become something of a game with him. Even if you become upset, that's reinforcing his behavior. The way to solve this is simply not to pay attention to him when he talks this way. You'll find that he may use the word "hate" a lot more for a day or so while he checks whether he can get you to react again. If you don't, he'll stop using it as much because it's no longer fun.