Yes, the Samsung PRO Plus 256GB SD is compatible with the Sony FX3.
The Samsung PRO Plus is a UHS-I U3 (V30-class) card. The FX3 allows a U3/V30 card for XAVC S HD and 200 Mbps recording, so it is not incompatible, but its higher-bitrate 4K modes need V60/V90, which this card does not meet. Prior record marked this incompatible; corrected to conditional.
Limitation
Cannot record the higher-bitrate XAVC S/S-I 4K modes -- they require a V60 or V90 card, this card is U3/V30. Works for XAVC S HD and the 200 Mbps recording modes.
“When the recording bit-rate is 200 Mbps, you can also record using an SDXC card (U3/V30)”
“UHS-I Speed Class 3 (U3) and Class 10 for 4K UHD and FHD video”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)