I'm angry about having to babysit my 3-year old brother
What makes you angry about having you babysit your own brother?
He refused to clean his mess, and that made me mad at him
You should be willing to help your parents when they need you to babysit.  Looking after your brother is annoying now, but it will help you to learn responsibility, which will help you out a lot later in life.
That's just it. I don't want to learn responsibility if it means babysitting my brother.
Your brother is only 3 which means that he needs a lot of help, and you are his older brother which means you are one of the most important people in his life. He needs you to help teach him how to be a better person, you are kind of like a hero to him. Isn't that cool?