Chances are a number did not get printed next to the "3" and it is from 2013. If it were from 2003, no doubt the feed would smell funky. Generally, they say that feed will last about 3 months after the use by date. Feed gets old and stale, and the nutrients start to break down. The color of the feed will fade as well when it becomes stale.
But all this being said, if the bag was stored properly, not left in an over heated shed, bugs or rodents were not allowed to have a field day in this bag, I would say to continue using it, as long as they are going to consume it relatively quickly. Don't keep feeding from this bag in another month or two. If you are too concerned, then go ahead and get another bag.