It looks like patreons get a fuller rule book for the Full-combat version of Grimdark Future that includes multiplayer rules (I have already had multiple requests to play with 3+ players). Does this also exist for Firefight?
In my experience, the most important thing in playing with an odd number of players (and if you don't want to split them into two teams, of course), is a non-square table. I've played a 3-players campaign of Frostgrave where almost every game all 3 of us were playing together on the same table. Table used was round, and we deployed at an equal distance from each other along its circumference. Worked like a charm.
Whilst the Full Rulebooks are technically for Grimdark Future and Age of Fantasy, since the low-model games have so much overlap you can use all of the advanced rules for Firefight and Skirmish as well.