MacRumors published a story that Apple has released the third betas of iOS and iPadOS 15 to registered developers.

Apple Seeds Third Betas of iOS and iPadOS 15 to Developers

Apple today seeded the third betas of iOS and iPadOS 15 to developers for testing purposes, with the updates coming almost three weeks after Apple released the second betas.

Registered developers can download the profile for the iOS and iPadOS betas from the Apple Developer Center, and once the profile is installed, beta updates will be available over the air. It's best to install iOS 15 and ‌iPadOS 15‌ on a secondary device because this is early release software that could have issues.

Apple Seeds Third Betas of iOS and iPadOS 15 to Developers