First to say that the camera module 3 is not supported by the current OpenFlexure software v2. It also does not fit the current v7.0.0-beta2 hardware.
Also you need to be clear that this is likely to break the focusing mechanism, it might break the whole camera.
If you are looking to use the camera with other hardware/software then I found that the lens unscrewed simply by gripping the front black roundish part with a pair of pliers. There was quite a lot of resistance to start with, it is probably glued in. Once started, it unscrewed quite easily. I have only tried on one example, so I might have been lucky.