I don't have the S3 in front of me, but I believe on the top of it is a button marked with a microphone. When in playback mode, hit this button to bring up the recorder. I believe set stars and stops it. I find it useful to set the review time to "Hold", which keeps the picture on your LCD until you cancel out of it (versus 2 seconds, for example). This way you can record your notes right away without having to switch to playback mode each time.
No experience with the Sony, but I wish they'd put this feature on SLRs.
By the way, you might get faster/more responses if you place your question in the correct forum (such as the Canon forum, where the S3 is a popular topic).
Cameras: 40D | SX10 IS
Lenses: 10-22 f/3.5-4.5 | 17-55 f/2.8 IS | 70-200 f/2.8L IS
Flash: 430EX | Sto-Fen Omni-Bounce
Accessories: BG-E3 Battery Grip | Extender 1.4x II | Hoya & Singh-Ray Filters