Huawei Freebuds 3 microphone does not work on Ubuntu 18.04
Apparently the problem here is that the Huawei FreeBuds 3 do not support Headset profile (HSP). They only support Handsfree profile (HFP) as can be seen in the bluetoothctl output. PulseAudio on the other hand does not support HFP out of the box, as is explained here: https://www.freedesktop.org/wiki/Software/PulseAudio/Documentation/User/Bluetooth/
So the only options are to install and setup oFono or to patch and compile PulseAudio yourself. Patch is available here: https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/94