We had a request to open specific inbound port (3000) because users have some web application server running on the machine.
Is that something that can be done (searched around and found that opening port is not something that is always working) and can it be scripted?
What issue are they running into that makes you believe that traffic on this inbound port is not being received? I would think that the web application server is what needs to be checked instead of macOS. Can you provide more details? There are a lot of very smart people here, so I'm sure someone knows a solution to this.