[MadGest]

var canvas = document.getElementById("canvas");
 var ctx = canvas.getContext("2d");
 var WIDTH = 400;
 var HEIGHT = 300;
function Box(ctx_, id_)
{
	this.ctx = ctx_;
	this.x = 75;
	this.y = 50;
	this.dx = 15;
	this.dy = 15;
	this.id = id_;
	this.dragok = false;
}

Box.prototype.create = function() {
	
	this.ctx.beginPath();
	this.ctx.rect(this.x,this.y,this.dx,this.dy);
	this.ctx.closePath();
	this.ctx.fill();
}
Box.prototype.cleare = function() {
	this.ctx.beginPath();
	this.ctx.clearRect(this.x, this.y, this.dx, this.dy);
	this.ctx.closePath();
	this.ctx.fill();
}
Box.prototype.getX = function() {
	return (this.x + canvas.offsetLeft);
}
Box.prototype.getY = function() {
	return (this.y + canvas.offsetTop); 
}
Box.prototype.isFocus = function(mX,mY) {
	if(mX>this.getX() && mY>this.getY() && mX<(this.getX() + this.dx) && mY<(this.getY() + this.dy))
		return true;
	else
		return false;
}

function getRandomArbitary(min, max)
{
  return Math.random() * (max - min) + min;
}

function clear() {
	ctx.clearRect(0, 0, WIDTH, HEIGHT);
}

function GetId(mX,mY)
{
	for(p in rect)
	{
		if(rect[p].isFocus(mX,mY))
		{
			return rect[p].id; 
		}
	}
}

function draw()
{
	clear();
	for(p in rect)
	{
		
		rect[p].create();
	}
}
function Move(e){
	var id = GetId(e.pageX,e.pageY);
		rect[id].x = e.pageX - canvas.offsetLeft;
		rect[id].y = e.pageY - canvas.offsetTop;
	
}
function Up(){ 
		
	 canvas.onmousemove = null; 
}

function Down(){
		
	canvas.onmousemove = Move;
}

var rect = new Array(3);

for(var i=0; i<3 ; i++)
{
	rect[i] = new Box(ctx,i);
	rect[i].x = getRandomArbitary(0, 300);
	rect[i].y = getRandomArbitary(0, 260);
	rect[i].create();
}
setInterval(draw, 10);
canvas.onmousedown = Down; 
canvas.onmouseup = Up;

Немного переработал скрипт. Есть баги небольшие, отладишь сам.