Yes, the Nikon Z6 III is compatible with the Samsung PRO Plus 256GB SD.
Samsung PRO Plus is UHS-I, 90 MB/s write per Samsung's own spec page. Nikon's official Z6 III manual recommends an SD card sustaining at least 250 MB/s for high frame-size/rate video (a raw benchmark floor, not a V-class label) -- 90 MB/s falls well short of that. The camera's standard 4K30 mode only needs 45 MB/s, which this card clears with wide margin.
Limitation
Cannot reliably record 4K60 or other high frame-size/rate modes -- Nikon's manual recommends an SD card sustaining at least 250 MB/s for those, and this card is rated at 90 MB/s. Works for 4K30 and standard video recording.
“UHS Speed Class 3 or better SD cards with a maximum data transfer rate of at least 250 MB/s, are recommended for recording and playback of videos with a high frame size or rate.”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)