Historical Rules · 26 August 2016
Rule 810.11
Official source text from the 26 August 2016 Rules release
The Two-Headed Giant variant can also be played with equally sized teams of more than two players. For each player a team has beyond the second, that team’s starting life total is increased by 15 and the number of poison counters required for the team to lose is increased by five. (These variants are called Three-Headed Giant, Four-Headed Giant, and so on.)