The easiest way: don't delete anything in Feedburner - just update the Original Feed URLs
on the two blogs that you are removing to use the same Original Feed as the blog you are keeping.
But check if any email subscribers are subscribed to 2 or 3 of the feeds and pause their
subscription for one of the feeds so they don't get duplicate feeds.
Also , you may not want to entirely delete the 2 blogs, but rather turn all posts into drafts,
post a link to the one blog you're keeping live, and setup Post Feed Redirect.