Apple has released iOS 4.3 Beta 3 called 8F5166b to developers for iPad, iPhone 4, iPhone 3GS, and iPod touch 3rd and 4th generation. A separate package is also available for the 2nd generation Apple TV.
iOS 4.3 betas allow cancelling app downloads and deleting apps while they're still downloading. This is not possible in iOS 4.2.1, and will be a welcome feature the next time you change your mind about installing an app mid-install.
As for new features specific to the third beta of iOS 4.3, there's no word yet.
[tags]iso 4.3,iphone 3gs[/tags]