https://wccftech.com/download-macos-big-sur-beta-3/

Apple has released the third beta of macOS Big Sur for download. Registered developers can grab the update over the air.

Those running the second beta of macOS Big Sur will be pleased to learn that beta 3 has arrived and can be downloaded over the air immediately. Just like any other early beta updates, this one features a load of bug fixes and performance related enhancements under the hood. If you have been experiencing lots of crashes or any other issues, there is a high chance that this update has fixed most of those issues. Since this is beta software, therefore you can still expect it to be rough around certain corners, but only slightly less compared to previous betas.