Apple released iOS 6.1 Beta 2 back in November and today they have released iOS 6.1 Beta 3 to developers registered in Apple’s iOS developer program. There isn't any information on what the newest beta release has to offer, but we expect that the Apple engineers would have eradicated some bugs and improved performance from the first two beta releases.
If you are registered developer, you can either download the standalone update or directly OTA (over-the-air) on your iOS device which has beta 2 already installed on it, by heading over to Settings > General > Software Update.
The previous beta release faced extreme secrecy with the main focus on giving developers improved access to use Apple Maps in their own applications. Apple hopes to boost the popularity of their mapping system by opening various APIs to developers.If you aren't already a registered developer and would like to get access to the latest iOS betas, you will have to register by paying the annual $99 fee for the developer program.
source - Apple