savepng3x.jsx

var pngName = prompt ("enter file name", "", "")

if (pngName) {
    var docRef = app.activeDocument
    docRef.selection.selectAll()
    docRef.selection.copy (true)
    const iPhone6PlusSize = 2208
    // create temporary document with maximum possible design size
    var newDoc = app.documents.add (iPhone6PlusSize, iPhone6PlusSize, 72, "temp", NewDocumentMode.RGB, DocumentFill.TRANSPARENT, 1.0, BitsPerChannelType.EIGHT)
    newDoc.paste (false)

    var folderPath = docRef.path+'/export/'
     var folder = new Folder (folderPath)
     if (!folder.exists) {
        folder.create()
     }

    var oldUnits = app.preferences.rulerUnits //save settings
    app.preferences.rulerUnits = Units.PERCENT;
    
    const fileExtension = '.png'
    // 3x
    var docPath = folderPath + pngName +'@3x' + fileExtension
    SavePNG(newDoc ,docPath)

    // 2x
    const scalePercent3xTo2x = 100.0/1.5
    activeDocument.resizeImage(scalePercent3xTo2x, undefined, undefined, ResampleMethod.BICUBIC);
    docPath = folderPath + pngName +'@2x' + fileExtension
    SavePNG(newDoc ,docPath)

    // 1x
    const scalePercent2xTo1x = 50
    activeDocument.resizeImage(scalePercent2xTo1x, undefined, undefined, ResampleMethod.BICUBIC);
    docPath = folderPath + pngName + fileExtension
    SavePNG(newDoc ,docPath)

    app.preferences.rulerUnits = oldUnits; // restore settings
    newDoc.close (SaveOptions.DONOTSAVECHANGES)
 }

function SavePNG(doc, saveFile){
    pngFile = new File(saveFile);
    doc.trim(TrimType.TRANSPARENT, true , true, true , true)

    var pngOpts = new ExportOptionsSaveForWeb; 
    pngOpts.format = SaveDocumentType.PNG
    pngOpts.PNG8 = false; 
    pngOpts.transparency = true; 
    pngOpts.interlaced = false; 
  
    pngOpts.quality = 100;
    doc.exportDocument (pngFile, ExportType.SAVEFORWEB, pngOpts)
}