Tony Hawk Pro Skater 3

Double check the IP-adress you've entered. This has to be the local IP-adress of your PC.
I assume you've entered that correctly, but checking it won't hurt

The IP address is correct. I know someone who knows how to do this correctly, I'll ask him.

Forwarded 5150, 5151, and 6500 for BOTH my public IP and local machine IP for both protocols. Windows Firewall is disabled, no other firewall software is installed. canyouseeme.org now says "connection refused" instead of "connection timed out." From what I've read "connection refused" means the server can see the port but nothing is responding to any inbound traffic on it. I started THPS3, started a server, tested all 3 ports, same result. I did this on both my PC and my iPhone.

Are these ports correct? portforward.com lists different ports for this game. It's like the program isn't listening over these ports.

Link me the portforwarding page that states different ports, all THPS users are using 5150 5151 and 6500, so I think you're looking at the wrong game.

Yeah you're right, probably clicked the wrong link on their site. I'm still wondering why this isn't working though.

It might sound stupid but in my router settings I've to manually enable them. It seems unlikely but it may be that they're unabled

Here's what else I tried:

Disable uPnP: Fail
Forward a random port below 1024: Fail
DMZ my machine: Fail

I've read that some ISPs will block ports above 1024. Tried forwarding a port below this for public and local IPs, still get "connection refused." So this either boils down to my router or my ISP. I'll have to jack in directly from the modem to know for sure. Will report back.

Direct from the modem still getting the same results. Come on, even common ports like 80, 21, etc. show up as blocked on various port checking sites. Am I screwed or can I call the ISP to resolve this? It's Charter, by the way.

You can try that. If it's known to them they might be able to help you straight away.
Good luck I hope you find the solution :)

im using router, those 3 ports are forwarded manually and all of those port checking sites say that ports are not available even when i'm hosting and ppl are in.

it's called "virtual servers" on my router where you manually forward ports needed in case if it matters.