Both are in beta-testing for new firmware releases
OP, to be clear, everything Fractal makes always has a stable release firmware and a beta firmware concurrently available. Your FM3 will ship with stable release firmware. Your participation in open beta firmware testing is voluntary. Should you choose to run open beta firmware, you can be sure that there aren't going to be any issues that will damage your unit. You should, however, take care to exercise some precautions:
1 - Back up working presets, blocks, and user cabs.
2 - Have a copy of the release (or previous beta) firmware in case you need to revert for whatever reason.
3 - Understand that beta firmwares often include new features but can sometimes "break" your presets due to changes in fundamental ways things work, leaving you to have no sound. Minor troubleshooting may be required.
4 - Functionality may disappear altogether. Parameters can become obsolete because subsequent innovations render them unnecessary, or they can be moved around in blocks so that the unit can run on more efficient code. This leads to decreased CPU usage, which allows you to use more bells and whistles.
5 - Patience is not only a virtue, but a requirement in the betaverse. One should not become huffy and petulant about features available on the flagship device (the Axe-FX III) that are not yet available on the FM3 or FM9. The FM's are by design less powerful, less expensive options and development is by nature an exercise in trickle-down economics. This is frustrating for some. Those features and capabilities may take a long time to be ported over, and some may never be dues to the constraints of the hardware.