Plug your K3 into the USB port on your computer, and copy your MP3's to the Music directory on the Kindle. Click Home, then Menu, then select Experimental, and select Play MP3's. Use the volume control on the bottom of the kindle to control the volume, and use headphones if you don't want to disturb others. Windows Media Player identifies the Kindle and will sync to it.
Alt - Spacebar will pause/start the music, and Alt- f will skip to the next track.
To access mp3's as "audible books", put them in the Audible folder instead of the Music folder. These will show up on your home page like any other book.