Yes, the Sony FX3 is compatible with the Sony SF-G Tough 128GB.
The Sony SF-G TOUGH is an SDXC UHS-II V90 card (write up to 299 MB/s). Its V90 rating meets the FX3's most demanding SD requirement (XAVC S-I 4K needs V90), so it records every SD-recordable mode on the body. Prior record marked this conditional; corrected to compatible.
“XAVC S-I 4K 600Mbps CFexpress Type A memory card (VPG200 or higher) SDXC V90 or higher”
“Class 3 (UHS Speed Class), V90 (Video Speed Class)”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)