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 not involved in those Combats or who don't already have Units at that Battlefield. See rule 447.2.a. for more information on Invalid Destinations.
§ 462.1 Core Rules PDF (officieel)
§ 462 Combat can only occur between Units controlled by exactly two players.
