My friend group often finds ourselves in this situation , quite regularly , while we gave 4-5 players people cancel or just cant make it
So we sat down one day at a pub and formalised some rules, which I wrote up in a document ... which i share here
https://docs.google.com/document/d/1f8mEhIE1AjRM-5xn-hDznBixeb2JlOVSNqi8k6DZKXU/edit?usp=sharing
TLDR version
4 Deployment maps balanced for 3 players, gameplay is based around king of the hill/take & hold, missions these can be played with 1 or more objective markers
but for simplicity sake use 1 unless you are using the dedicated 3 relic deployment zone for this map moving off the left or right of the board results in a Pac man style appearing
on the opposite side .... helping balance the board a bit better . ( Presume your fighting in the warp on some demon planet )
Scoring
Points are scored at the end of at the 3 player battle round Ie the last persons turn
The scoring system means that winning the first turn and rushing out to take an objective early is going to get you targeted pretty quickly by both players heavier firepower and to combat some of this we considered banning advance deployment, instead, we gave those units the standard deep strike rule instead .. Using the beta rules this feels fair ... otherwise, the person who deploys -2 to hit Eldar rangers on the objective before the start of turn 1 is gonna win ... (I hope your playgroup immediately gang up on them for breaking whats supposed to be a casual format )
The second way of scoring points relies on you spreading the pain around rewarding killing a unit from each player, Scoring 1 point ( Max) if you manage to kill a unit from each player
inside the battle round, this again is also going to make you a target and can be tricky to pull off we play smaller games in this format for speed and sanity sake .... (ever tried to fit 2k swarm Guardsman on a very small deployment zone )
We have also bought in a
CP spend system focused on creating interesting team-ups, blackmails and bribery we'd do this is anyway but it makes for a fun game and gives the space wolf player somthing to spend
CP on ...