You need to edit the respective game.xml to be found in the subfolder “games” of your TripleA map.
Best idea would be to study the xml of existing FFA-mods and compare what is different to “regular” game.xml.
Take as an example Napoleonic Empires 5 Player FFA:
If a Japanese industrial complex is built on a Chinese territory
and that territory is later recaptured by the Chinese or liberated
by another Allied power, the industrial complex is removed from the game.
I took my 1999 Europe, 2001 Pacific, 2004 Revised, 1942, 1942 2nd edition, and 1941 and put them into Artbins. Like you, I left specific campaign, global, and AA50th in their boxes.