promiseall.js

var files = req.files;
var promisarr = [];//initialize promises array
function multiplefiles3(resobj) {
  console.log(resobj.originalname);
  var params = {
    Bucket: "******",
    Key: resobj.originalname,
    Body: resobj.buffer
  };
  return s3bucket.upload(params).promise();
}

var uploadsize = 0;
files.map(file => {
  uploadsize = uploadsize + file.size;
  promisarr.push(multiplefiles3(file));//push individual promises to that array , i'll refer them as child promises
});

Promise.all(promisarr)//evoke Promise.all() function , i'll refer this as parent promise 
  .then(result => {
      //this result is a array of objects from the fulfiled promise
  })
  .catch(err => console.log(err));//if any of those promises get reject this whole promise will be rejected