Creating a single aggregated RSS feed for multiple blogs

I am going to have 3 separate blogs, but need to have a single RSS feed that includes the posts from all 3, sorted by date posted (so it would be a single, combined list of posts from several blogs).

Is there a way (or a plugin) to do this? I’ve seen plugins that aggregate multiple RSS feeds into a single html page, but I need to take multiple RSS feeds and combine them into a single RSS feed.

Thanks.