A lucky Reddit user by the name of GroveStreeHomie was able to buy a (currently unannounced) third-generation Chromecast from his local Best Buy. The new device features a slightly tweaked design and might have some new features, such as Bluetooth. Unfortunately, the device is currently unusable as it requires an update to the Google Home app that isn't available yet.
https://www.notebookcheck.net/A-Best-Buy-customer-accidently-bought-the-unannounced-thid-gen-Chromecast.336522.0.html