Arena matchups are based on your arena rank score ("PT", though I'm not sure what that stands for), not your character power rating.

You should be able to mostly beat players whose power is comparable to your own by choosing the matchups to give you an elemental advantage. The 3 on 3 battle basically starts as three 1 on 1 duels, and only if one character wins her duel does she go assist her teammates. For example, I'm currently on a 22 game winning streak, but probably had a lower total power rating than my opponent in about half of those matches. In most of them, I'd have lost had my opponent gotten to choose the pairings.