440. Combat can only occur between Units controlled by exactly two players.

440.1. In Modes of Play with more than two players, Battlefields with Staged Combats or Combats in Progress are Invalid Destinations for Moves of all kinds ( Standard Moves or otherwise) by Units controlled by Players who don't already have Units at that Battlefield. See_rule_REF_425.2.a_DOT for more information on Invalid Destinations.

440.2. In Modes of Play with more than two players, Battlefields with Staged Combats or Combats in Progress are Invalid to be chosen as a location to play one or more Units by a player not involved in that Combat by any means,

440.2.a. If an effect would require a Unit be played to a Battlefield with a Staged Combat or a Combat in Progress , where the controller of the played unit is not a participant, instead the Unit is played to its controller's Base.
440.2.b. Any subsequent reference to "here" in the corresponding effect is reassigned to the Controller's Base , where the Unit was played. Any further effects that may be invalidated are invalidated as if the effect was mistargeted. See_rule_REF_352.5_DOT Targeting for more information on Mistargeting.

440.3. All choices that would result in a Combat occurring between more than two players simultaneously are invalid and ineligible to be completed.