Yes, the Lexar Professional 2000x 256GB SD is compatible with the Sony FX3.
Sony FX3's SD slot accepts Lexar Professional 2000x 256GB SD directly. V90 class clears the SD slot's V90 floor required for the camera's top-tier XAVC S-I 4K/HD intraframe modes (600Mbps/222Mbps) — compatible for every recording mode on this slot.
“XAVC S-I 4K 600Mbps CFexpress Type A memory card (VPG200 or higher) SDXC V90 or higher”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)