Normally, "an" + noun is general and the identity of the noun is NOT known to the listener. "The" + noun is specific and the identity of the noun is known to the listener. In this example, the number- 3%- is not known to the listener until the speaker tells him. In everyday English, this is one of those things that you begin to say correctly after lots and lots of listening practice. Please keep in mind that even if you get them mixed up when you speak, it's ok! The difference is minor and everyone will still understand you when you speak :)