Hi! The fact that you see those Legendaries in Bronze 3 means that despite having those Champions, for some reasons, those players can't advance further or were defeated in higher Tiers. From that, you can assume that those accounts don't harness those Champions to full their extent. If so, there are decent chances you can beat them as well. However, it would be safer to say so if we could see your Collection of Champions.
Also, I would like to remind that we're actively preparing some changes for the Arena matchmaking algorithm that will make progression through Bronze and Silver Tiers a bit easier. We cannot provide you with a certain date for now, but more news are likely to appear in the next few weeks