The iPhone Dev-Team has already confirmed that the Redsn0w 0.9.8 (Windows/Mac) for iOS 5 can jailbreak the Beta 3, another iPhone hacker ih8sn0w has confirmed that a Sn0wbreeze update is available that will support jailbreaking on that particular build.
Remember, this is a tethered jailbreak for all devices except for old bootrom iPhone 3GS. iPad 2 isn’t supported. Also, this is intended to be used by developers only.
You can download iOS 5 beta 3 from here, and iTunes 10.5 beta 3 from here (required for iOS 5 beta 3).
Sn0wbreeze:Download Sn0wbreeze 2.8b4 for Windows
Redsn0w:Download Redsn0w 0.9.8b2 for Windows
Download Redsn0w 0.9.8b2 for Mac
Source 1, Source 2