Yes, the Samsung PRO Plus 256GB SD is compatible with the Sony Alpha A9 III.
Samsung PRO Plus 256GB SD is V30 class. Clears: XAVC S HD (100 Mbps, requires U3). Does not meet the floor for: XAVC HS 4K (280 Mbps, requires V60); XAVC S 4K (280 Mbps, requires V60); XAVC S-I 4K (600 Mbps, requires V90); XAVC S-I HD (222 Mbps, requires V90).
Conditions for compatibility
Compatible for stills and XAVC S HD (100Mbps, U3 met). Cannot record XAVC HS 4K (280Mbps, requires V60 — UHS-I bus cannot achieve V60), XAVC S 4K (280Mbps, requires V60), XAVC S-I 4K (600Mbps, requires V90), or S&Q Motion (requires V90). Full HD only for video.
Limitation
UHS-I V30 card. 4K recording not possible -- XAVC HS/S 4K require V60 minimum. Intra-frame recording not possible -- requires V90. Full HD (XAVC S HD, 100Mbps) only for video.
“XAVC HS 4K / XAVC S 4K SDXC V60 or higher XAVC S HD SDHC/SDXC card (U3 or higher) XAVC S-I 4K / XAVC S-I HD SDXC V90 or higher”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)