This has been discussed at length. Nobody knows for sure but since Activision is the current holder of the Seirra On-Line branding and IP, it's a safe bet they own the IP but just don't care about it.
Best way to build an OP3 is to just start. That's something that's been tried many times and has failed many times (see
this sub forum for the list of projects that came and went). I'm not saying it's impossible to make a fan made OP3, just that there are no dev teams here that have the time to devote to it. The most active projects ATM are
my own, some updates to OP2 mission building that Arklon/BlackBox are working on (unannounced, I don't have a lot of details just that it's really interesting) and the
OP2 API efforts that Vagabond and Hooman have been working on (multiple topics in that sub forum). Lord Palandus has been working on a few interesting projects unrelated to Outpost (
here). Also Zhall was working on a
mod to SC2 (I think?) that looked interesting. I think he's stated it's finished but it could be worth checking out.
As for discord, no official channel. I've been pushing for it but the other mod's are... aren't as enthusiastic about it as I am. On the other hand, we do have an IRC channel. You can join using an IRC client your you can use the Chat link at the top of the page to use our web-based IRC client.