At the moment Filmic does not support the DJI Osmo Mobile 3 not though any fault of Filmic but because of DJI having a buggy SDK.
Below is my responce from Flimic.

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Thanks for writing in! We're pretty excited about the Osmo Mobile 3—it seems like a pretty great gimbal! Response to the new gimbal has been huge so we are very likely to create custom integration for it, we just haven't done it yet and I don't have an exact date of when it might happen. DJI recently released a beta version of their SDK for the Osmo Mobile 3, but it did have significant bugs so we'll need to wait for them to fix that and issue a final SDK release before we can create our own integration.

Out of the box at the moment you can use the Osmo Mobile 3 for basic stabilization, and it may be possible to start/stop recording from the gimbal. To do so, follow these steps:

1.) Turn on your Osmo Mobile 3.
2.) In your System Settings, select Bluetooth and connect to the Osmo Mobile 3.
3.) Do NOT turn on the DJI Osmo Mobile toggle in FiLMiC Pro's Hardware settings. If it's on, toggle it off.
4.) In FiLMiC Pro's Settings > Device, scroll all the way to the bottom and toggle "Volume Keys Trigger Recording" ON.

Once you've done this, you should be able to use the record button to start/stop recording. Note that as we have not actually done any work to create specific integration this functionality is not officially supported, but I was able to get it working on a test unit!

Other than that, FiLMiC Pro is proud to support best-in-class, custom integration for a variety of other smartphone gimbals. These gimbals include the Zhiyun Smooth 4, Freefly Movi Cinema Robot, as well as DJI's Osmo Mobile 2 and Osmo Mobile 1. To learn more, please refer to the following article: Which Gimbals are Currently Supported by FiLMiC Pro?

Please let me know if you have additional questions and I will be happy to assist!

Hello and good day grahamjohnson10. Thank you for sharing these information. This is well detailed explanation and thank you for your support.

There's a thread already running on this subject in which everything you mention has been reported - see https://forum.dji.com/forum.php? ... D710%26typeid%3D710

The SDK is a beta, as you mention.  Betas are released to enable bugs to be spotted, reported, and fixed in the final release.  Hopefully that won't take much longer.