`three = 1` in the linux sourcecode
Read the comment above ext4_list_backups right above:https://github.com/torvalds/linux/blob/d158fc7f36a25e19791d2...    /*
     * Iterate through the groups which hold BACKUP superblock/GDT copies in an
     * ext4 filesystem.  The counters should be initialized to 1, 5, and 7 before
     * calling this for the first time.  In a sparse filesystem it will be the
     * sequence of powers of 3, 5, and 7: 1, 3, 5, 7, 9, 25, 27, 49, 81, ...
     * For a non-sparse filesystem it will be every group: 1, 2, 3, 4, ...
     */

I'm not sure what's so noteworthy about that. There's ton of arcane code in any kernel, as long as it's commented correctly there's no issue.