freecodecamp : sorted union

unite.js

function unite(arr1, arr2, arr3) {
  var combined = arguments[0];
  
  for(var x = 1; x < arguments.length; x++){   
    for(var i = 0; i < arguments[x].length; i++){
      var counter = 0;
      for(var j = 0; j < combined.length; j++){
        if(arguments[x][i] != combined[j]){
          counter++;
        }
      }
      if(counter == combined.length){
        combined.push(arguments[x][i]);
      }
    }
  }
  return combined;
}

unite([1, 2, 3], [5, 2, 1, 4], [2, 1], [6, 7, 8]);