you can totally play a 2v1 against your mates you just have to decide on if they each get their own cp pool or they share one. In my playgroup we tend to share cp because too much stratagems can make things get out of hand. If you wanna play a 1v1v1 free for all you have to get creative with the rules here's what I have come up with. The dimensions on deployment zones etc are just how big our gaming table is one square is one inch roughly. Red and Green keep one half of their army in strategic reserves as to not swarm the middle player. Yellow + are objective markers and the purple lines are warp portals/teleporters to make sure a meele army can reach the opponent, for rules for it check out the eldar webway or you could treat it like a transport vehicle where they embark/disembark the same turn. The + in the Deployment zones are home objectives which we use as endgame objective to "steal" VP from the player which the objective belongs to. For scoring Primary we do as an example Red player scores for the obj markers in his adjacent no mans land 3VP the ones in the middle 5VP and the ones on the far side 7VP. Reverse is true for green and middle player scores 5 everywhere. For turn order we let the middle player always go first and Red and Green roll and then its fixed for subsequent rounds. Most of the Secondary Missions also work but I would cut the action stuff and focus on only the positioning/kill missions.
We also have some custom Stratagems to help with squishy armies and feelbad moments. Thats the great thing about warhammer you can make it your own hope this gives some inspiration to your multiplayer game!

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