No, the Lexar CFexpress Type B Professional Gold 128GB is not compatible with the Sony FX3.
Sony FX3 Slot 1 is CFexpress Type A only. Sony's own Help Guide explicitly states CFexpress Type B memory cards cannot be used — Type B uses a physically different, larger connector than Type A.
What works instead
See all compatible memory card for the Lexar CFexpress Type B Professional Gold 128GB →“CFexpress Type B memory cards cannot be used.”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)