My post from December got outdated in the meantime.
Currently the download pages for stable and dev builds offer the same file format, for installation with fastboot (the filenames starting with “IMG” serving as an indication).
https://doc.e.foundation/devices/FP3/install is correct currently in this regard.
(If preferred, install files suitable for installation with ADB sideload can be obtained by using download links listed in the “url:” lines in https://ota.ecloud.global/api/v1/FP3/stable and https://ota.ecloud.global/api/v1/FP3/dev. How to install these is still given in https://doc.e.foundation/devices/FP3/upgrade, but this is not deliberate I guess, they seem to have forgotten to update the page.)
Actually, on the stable release channel there is …
"OS OTA upgrades
Status: 
Complete
Completed
[…]
- FP3 >> R (A11) to S (A12) stable"
(Week 19, 2023: Development and Testing Updates - Development Updates - /e/OS community)