HI Daniel,
In theory 301 redirecting should pass the link juice directly to your new URL, but if the URL has never been indexed by Google, those 301's may take a period of time to get indexed and reflect. This is really a subject matter, I have heard 301's and site redirects working very quickly (like same day), and other times it has been a couple months. I "feel" like this has a lot to do with the sites original traffic and Google crawl rate. For example when SEOMoz became Moz, they were re-ranking relatively quickly. While sites I have worked on have in fact take several weeks to return to the SERPs.
What that really means is it can take awhile for Google to re-evaluate your new URL's, likely on their next index pass of your site.
It is also very important to note that just because your "old url" ranked in the top 3 does not mean the new URL will rank in the top 3. Google will re-evaluate the new url and place it in the appropriate spot. It does of course consider the link juice that your old URL had, but that does not necessarily mean it will rank the same. There are a lot of factors that go into the way page ranks.
I hope this makes sense and helps,
Don