Add @3x resolution to existing Sketch documents

Add @3x.sketchplugin

// This plugin will add a @3x size to all existing slices and exportable layers,
// that already have a @1x and @2x size

var p = [doc pages]
for (var i = 0; i < [p count]; i++) {
  var current_page = [p objectAtIndex:i]
  var exportable_layers = [current_page exportableLayers]
  for (var l = 0; l < [exportable_layers count]; l++) {
    var slice_or_layer = [exportable_layers objectAtIndex:l]
    if ([slice_or_layer exportOptions]) {
      var has1x = false,
          has2x = false,
          has3x = false

      var opts = [slice_or_layer exportOptions],
          sizes = [opts sizes]

      for(var k=0; k < [sizes count]; k++) {
        var size = [sizes objectAtIndex:k]
        if (size.scale() === 1) {
          has1x = true
        }
        if (size.scale() === 2) {
          has2x = true
        }
        if (size.scale() === 3) {
          has3x = true
        }
      }

      if (has1x && has2x && !has3x) {
        // Let's add a @3x size
        var firstSize = [sizes firstObject]
        var newSize = [opts addExportSize]
        [newSize setFormat:firstSize.format()]
        [newSize setScale:3]
        [newSize setName:"@3x"]
      }
    }
  }
}

// This is just a hack to force the updating of the UI to see the new sizes in the inspector
[[doc currentPage] deselectAllLayers]