Yes, the SanDisk Extreme microSDXC UHS-I 128GB is compatible with the Sony FX3.
Sony FX3 has a full-size SD card slot. SanDisk Extreme microSDXC UHS-I 128GB (real write speed 130 MB/s, V30 class) fits via the bundled microSD-to-SD adapter and clears XAVC S HD (non-4K) recording, but does not meet the Video Speed Class 60 floor this camera's 4K modes require via the SD slot.
Limitation
Cannot record any 4K mode via the SD slot -- FX3's own spec requires Video Speed Class V60 minimum for every 4K mode (XAVC HS/S) and V90 for XAVC S-I, this card is V30. Works for XAVC S HD (non-4K, UHS Speed Class 3 floor, met) only. The bundled microSD adapter provides physical fit only; it does not change the card's class rating.
“aximum recordable bit rate Supported memory card XAVC HS 4K 280Mbps CFexpress Type A memory card (VPG200 or higher) SDXC V60 or higher XAVC S 4K 280Mbps CFexpress Type A memory card (VPG200 or higher)”
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