Any of Doom 1 2 or 3 doable in coop? If so how?

Short answer, no. Long answer, from what I can tell DOOM 3 only had co-op on the Xbox, the PC never had native co-op support. There was a mod available for the original version of DOOM 3 but I cannot confirm if the mod works with the BFG edition of the game.

More to the point however, the DOOM 3: BFG Edition sold on GOG does not support multiplayer at all. You might be able to get some kind of peer-to-peer system working with mods, but natively you are out of luck.

This is terrible news. I did coop with Doom 3 on the original Xbox, and Doom 1 and 2 on 360. It's a great experience. They really need to do this for PC.

Just read this on the PCGamingWiki:

Despite the GOG.com version being single-player only. It is possible to play multiplayer by adding

seta net_useplatformbackend 0

in default.cfg and use the console command connect in order to join the server.

I really enjoyed using OpenCoop mod for Doom 3 (and Resurrection of Evil), however it will not work with the BFG edition as far as I know:/

So I realize I'm late to the party, but when I follow these instructions (after enabling the console) it tells me connect should be followed by an IP address. Any idea how I would find the IP address of a Doom 3 multiplayer server?

Being as I've never tried to play Doom 3 multiplayer or co-op I don't know exactly. However, you could always try pinging the server (if you know the server's DNS name). Or it could be that this fix on the PCGamingWiki is for LAN co-op only, in which case you'd know the IP address of the hosting server as it would be the other computer on the network.

ZDaemon too and you'll probably find more wanting to do the IWADs there. I've seen Co-Op be done via Odamex or Chocolate Doom too on Doomseeker, but Zandronum is by far the better experience of the lot with 3.0.

This however, should be done with the original Doom 1/2/Final Doom/Master Levels/what have you. BFG's only use community-wise for the original two is NERVE.WAD anymore.