Yes, the Sony FX3 is compatible with the Transcend 700S V90 64GB.
Sony FX3's SD slot accepts Transcend 700S V90 64GB 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)