I know you're worried about this being a heart issue. The thing is at three, he is just barely, barely old enough to understand how his actions affect others. It's not that he is especially unloving or that you've missed teaching him an important lesson, or that if you find the right words he'll get it and suddenly quit acting this way.
That's why the pp's are saying it's a phase and that the answer is to continue correcting his behavior and teaching him how to treat his brother and eventually this phase will pass, he will become cognitively able to empathize, and you'll start to see the fruits of your teaching.