Juror #10 reveals his prejudice almost immediately when he says, "Well, it's the element. They let the kids run wild. Maybe it serves ‘em right." By using the pronouns "they" and "them" he makes a general statement about the people who are considered poor.
Juror #3's prejudice is first revealed when he says, "It's the kids. The way they are—you know? They don't listen." He goes on to talk about his son and how he "made a man out of [him]." His prejudice is directed towards teenagers and their supposed lack of respect for their fathers.