If Parcelforce request address line 3 even when it has already been filled in then you will need to select a county for the shipment.
This is because the county is used for the 3rd address line so if the county field is empty then the error stating 'the 3rd address line is required' will appear.
As long as something is in the country field the error should not appear.
If this doesn't work or you need further assistance please get in contact with our support team.