I downloaded the stable version of the AR750s 3.212 firmware and it has the file name:
openwrt-ar750s-3.212-0407.tar
This is the exact same file as 3.212 beta 3 version of the AR750s firmware. Is there no difference between the beta 3 release and the stable firmware release?
But here is how it works: firmware are pushed as snapshot or beta. Beta firmware goes to testing. Once test passed it will be moved to release directly.
I just checked and the beta 3 file I downloaded on 12-Apr-22 matches the current stable released file, in both name and checksum. Today I downloaded the AR750 beta 3 file, again, and it has the same name and checksum as my other two downloaded files, but the checksum listed on your download page for this beta 3 version seems incorrect. I think there is something wrong with your download page for the beta 3 tar file, or maybe for the stable release. I am concerned with checksums don’t match what is stated.