Using flat() to flatten from multiple arrays into 1. Using Set on flat array to only get unique values.

Sorted Union

function uniteUnique(arr) {

  var newArray = [];
  
  for (let array of arguments) {
    newArray.push(array);
  }

  var flatArray = newArray.flat()
  var unique = [...new Set(flatArray)];

  return unique;
}

uniteUnique([1, 3, 2], [5, 2, 1, 4], [2, 1]);