I don't know what game_switch [X, Y, Z] is, but assuming that what you mean is that you want the zoom to be normal if in-game switches X, Y, and Z are all ON, then you could do that by changing the following line:
if @character.is_a?(Game_Player) and $game_map.isworldmap
to:
if @character.is_a?(Game_Player) && $game_map.isworldmap &&
$game_switches[X] && $game_switches[Y] && $game_switches[Z]
where X, Y, and Z are the IDs of the switches. If that isn't what you meant, you will need to clarify.