Let me just start off by noting that SuperSU v2.74, as well as the previous version 2.72, are fully compatible with Android N Preview 3. So if that is all the information you were looking for, there you have it.
This update brings nothing more or less than compatibility with Android N Preview 3.
Today's FlashFire update brings a major new feature and many smaller fixes and improvements.
This is a relatively minor update. Support for the patching ChromeOS-wrapped Android boot images has been added (Pixel C), some logcat and fsck issues were fixed in the SELinux policy, and some flags have been added to the ZIP installer.
The major new feature of today's FlashFire update is the ability to create backups that can be flashed back to the device with fastboot. This will allow you to restore your phone from your computer without booting Android or even a custom recovery. This is a new and experimental feature, and as such there are a number of ifs, buts, snags, and caveats.
Most of todays update is compatibility related (again), fixing issues with various popular update ZIPs.
Really isn't much to mention about this, other than that all the changes for N Preview have been merged with the main BETA releases, where they were two separate releases before.
Another FlashFire update mostly improving compatibility.
Today brings a minor update to CF.lumen, fixing some issues for Tasker users and fixing a minor performance snafu.
Today's update of FlashFire is mostly a compatibility update. A bug in Android N itself that prevented OTAs from flashing has been worked around (you may still see incorrect size listings of 2048 MiB in some places, though), and there have been quite a few modifications to partition detection and external storage handling (portable and adopted sd cards, as well as USB).