Have you configured serial port according to this Serial Port Configuration | openHAB port-configuration? That could explain the permissions issue
Please be aware of known issues with serial devices with openhab3 (regression in serial library used by openhab) Serial ports getting blocked after some re-connecting · Issue #1842 · openhab/openhab-core · GitHub. See also discussion in Modbus Binding not working on OH3. I am not aware that this would be any better in 3.0.1 unfortunately