I'd recommend getting at least one CFexpress Type A card (Sony or LEXAR) and a couple of high-quality V90 SD cards like the SanDisk Extreme PRO. The FX3 handles most common shooting modes fine with good SD cards, but having a CFexpress gives you headroom for those unexpected situations where you suddenly need 4K120 or higher bitrates. Depending on your typical shooting style, you might find you rarely need the CFexpress.

I've been using the Sony TOUGH CFexpress Type A cards with my FX3 since day one. Yes, they're expensive, but they've never failed me even when shooting 4K 120fps.

Honestly, I have to respectfully disagree with the sentiment that you need to stick strictly to the household names like Sony or SanDisk to get reliability. I've been doing a lot of market research lately and looking at what actual cinema houses are running, and tbh some of the more specialized brands are offering way better stability for the FX3 right now. From a market perspective, ur often paying a massive premium for the brand name rather than the actual flash memory quality. If you want my take on where the market is actually heading for high-speed video:

• Basically just go with ProGrade. You cant go wrong with them because they focus specifically on professional video workflows and their controllers are sooo much more consistent for long shoots.
• Look into anything from Delkin. They have a massive reputation for being rugged and their customer support is way better than the big tech giants if a card ever acts up.
• Honestly, any high-speed card from Angelbird is a solid bet. They are really geared toward the cinema crowd and their sustained write speeds are basically perfect for the FX3 bitrates. You dont necessarily need to overpay for the big logos as long as the internal tech is solid. Just my two cents after seeing how these brands have evolved lately!

Totally agree with Davzoweby about the brand tax. It is definitely a thing, but I want to throw a quick warning out there for people looking at the budget brands. While you dont need the most expensive cards on the market, avoid the absolute bottom-tier generic stuff you find on big marketplace sites. I have seen some of these off-brand cards claim high-speed ratings but they fail to maintain the minimum sustained write speed required for the FX3 bitrates. The last thing you want is the recording interrupted message right in the middle of a shoot because the card couldn't keep up. Also, keep an eye on heat... some of the cheaper high-speed cards tend to run way hotter than the pro-grade ones, which can actually cause the camera to overheat faster during long 4K sessions. Just make sure whatever you get has a solid reputation for sustained performance, not just peak bursts.

^ This. Also, 777CreatureTTV is spot on, but honestly even the so-called reliable big brands feel like a massive gamble lately. It drives me crazy how much we have to fork over for CFexpress Type A cards... it's such a scam when you look at how much cheaper Type B is for other systems. Companies just dont care about the end user anymore, they just want to lock us into these overpriced ecosystems. Honestly its ridiculous that you spend five grand on a rig and still have to worry about the media flaking out because quality control is going down the drain. Ive tried to be so careful and research everything to build a stable kit myself, but it feels like the manufacturers are just working against us. It is just exhausting. The whole situation is just a mess and it feels like we're being squeezed for every penny while the hardware gets flimsier... just sucks.

For 4K 60fps recording on the Sony FX3, high-speed UHS-II SD cards (V90) are sufficient. Recommended options include the Sony SF-G Tough UHS-II SD Card (300 MB/s read, 299 MB/s write) and the Kingston Canvas React Plus UHS-II SDXC Card (300 MB/s read, 260 MB/s write).