Did you try Direct Connection using an IP address?
The other options would be LAN (using VPN software, such as Hamachi or Tunngle) or Discord invites.
Can you join the other two friends, if they start a new game or (if available) load an existing save?
Are you shutting down all non-essential programs before starting the game, especially anti-virus and anything network related (utility or monitoring programs, etc)?
Have you tried joining anyone else?
Have you tried hosting/joining an Arena match?
Try going into Arena mode, create an arena, open the Connectivity menu and check the settings, then exit to the main menu and try joining. Alternately, create a multiplayer game lobby, exit and try joining.
If you are using any mods (though it doesn't sound like it), try testing in a new profile without any mods enabled.