Unfortunately, there is no way to aggregate iterations on the dashboard as you suggest. However, instead of using 3 different iterations, one per team, you could have all teams work in the same iteration and use the Team field on the Story or Bug to assign items to teams. But, if you do it this way, you would lose the per team burndowns. You may have already considered doing this. We use this approach for the 2 teams on the TeamPulse project. It works quite well for us as we feel the overall cross-team iteration burndown to be the most important.
All the best,
the Telerik team