after all these years why so difficult to create local LAN for 3 players in gog Ultimate doom
The menu does not even offer local LAN.

I have read various posts from forum and elsewhere but nothing clear to create and other player to join

I am happy to read posts on how to but not managed to find anything definitive yet - clearly I am not looking in right place an my two kids think I am a duffer :(

(worse, many years ago I set this up on college throw out 286 PC,s with twinax cable IPX network, from scratch on my own)

Last time when I was setting a multiplayer game I used Chocolate Doom source port, and there you create and join a multiplayer game in the setup, not in the actual game. This could be working similarly in the original game, so I would try looking there first.

https://www.dosbox.com/wiki/Connectivity#IPX_emulation

But I'd too rather adviced to use more advanced source port for network play. Like Chocolate, Crispy or PrBOOM+ if you want to go a puristic way. Or Zandronum if you are not too deep into purism.