If you're looking for a great monitor to pair with your Sony FX3, I highly recommend checking out the SmallHD INDIE 7 Touchscreen On-Camera Monitor. This 7-inch monitor is an excellent choice for the FX3 for several reasons:

1. Bright, High-Resolution Display: The INDIE 7 features a bright 1500 nit touchscreen with a resolution of 1280x800, providing a clear and detailed image for focusing and composition.

2. Essential Focus Aids: When shooting in 4K with the FX3, nailing focus is crucial. The INDIE 7 offers peaking, zoom, and false color tools to help you achieve critical focus with ease.

3. Exposure and Color Monitoring: The monitor also includes waveform, vectorscope, LUT support, and anamorphic desqueeze features, allowing you to accurately monitor exposure and color while shooting.

4. Lightweight and Durable: Constructed from lightweight polymer plastic, the INDIE 7 is easy to mount directly on top of your FX3 using 1/4"-20 mount points. It can also be used with a camera cage or rig for added versatility.

Overall, the SmallHD INDIE 7 is a feature-packed, high-quality monitor that perfectly complements the Sony FX3. Its bright touchscreen, focus aids, and exposure tools make it an invaluable asset for any FX3 shooter.

I use this one:

Atomos Ninja 5.2" 4K HDMI Recording Monitor: https://www.amazon.com/Atomos-Ninja-HDMI-Recording-Monitor/dp/B0CFPF7TJ5/

Atmos Ninja V is a cheaper option, and also best selling camera monitor.