oh I have tried updating my machine from Beta 1 to 2&3 at least 10 times .. but still it shows 20A4300b and pops up saying new update available. Let me try to reinstall it completely as you suggested.
But how did you install beta 3, can you pls share the steps?
In System preferences > Software Update popup I still see the same upgrade button which downloads 12 GB, restarts multiple times and then the same error.... I have tried it 10 times...