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.
§ 447.2.a Core Rules PDF (officieel)
§ 447 Moving is defined by the Origin and Destination of the Permanent that is changing locations.
§ 447.2 The Destination is where the Permanent is going to.
