application.js

var div1 = $("#the-div1");
var div2 = $("#the-div2");
var div3 = $("#the-div3");
var aziScroll = new AziScroll();

aziScroll.addEvent(1000, div1, function(element) { element.addClass("x") });
aziScroll.addEvent(2000, div2, function(element) { element.addClass("y") });
aziScroll.addEvent(3000, div3, function(element) { element.addClass("z") });

var onScroll = function() {
  var positionTop = $(this).scrollTop();
  aziScroll.onScroll(positionTop);
};

$(document).bind("scroll", onScroll);

scroll.js

function AziScroll() {
    this.events = [];
};

AziScroll.prototype.addEvent = function(xPosition, element, callback) {
  var event = new AziScrollEvent(xPosition, element, callback);
  this.events.push(event);
};

AziScroll.prototype.onScroll = function(positionTop) {
  var event;

  for(var x = 0; x < this.events.length; x++) {
    event = this.events[x];
    if(positionTop >= event.xPosition) {
      event.happens();
    }
  }
};

function AziScrollEvent(xPosition, element, callback) {
  this.xPosition = xPosition;
  this.element = element;
  this.callback = callback;
};

AziScrollEvent.prototype.happens = function() {
  this.callback(this.element);
};