there is no jailbreak for 3.0 beta 5 yet but you can downgrade to beta 4 and jailbreak and activate it using quickpwn (unofficial) 3.0 beta 4 this won't be an issue since your using iPhone 2G. Then you can patch out expiration by going to /System/Library/CoreServices and Modifying SystemVersion.plist by Removing the Key 'ReleaseType'
Note: you can upgrade to beta 5 when jailbreak for it comes out... Hope this helped...