The mp3 audio format is layer 3 from the Moving Pictures Experts Group (MPEG) 2 audio standard. Because of the popular use of the term MPEG 3 there is not and never will be an true MPEG 3 standard.
It is is a widely used standard, but it carries the burden of patents with royalties demanded for encoders and decoders. This makes it difficult for most free software developers to incorporate encoders and decoders, if they live in countries that recognize these patents.
Intellectual property law is a good thing when it encourages innovation and a bad thing when it stifles it. Cracking that nut is not so easy, but we seem to be moving in the wrong direction in the United States.