Yongnuo YN320EX Flash Speedlte 1/8000s for Sony a7 a7ii a6000 A7R-II A7R a6300 a6500

Q: Compatible with Sony a7 3

Yes, it can work on Sony a7R III.

Q: Does this work with Sony a6100??

Yes, it can work with Sony a6100.

Q: How does one check what firmware version of the flash?

Long press the function button [Zm/Fn] to enter the custom function setting option, short press [Up][Down] button to select custom items, short press [Left][Right] button to set the parameters of the items. Fn 10 vE.XX shows the version information of the flash firmware.