function print() {
var canvas = document.getElementById("mycanvas");
var img    = canvas.toDataURL("image/png");
document.write('<img src="'+img+'"/>');
}

var canvasMain = document.getElementById('canvasMain'); //destination canvas
var ctx = canvasMain.getContext('2d');

var canvas1 = document.getElementById('canvas1');       // layer 1
var canvas2 = document.getElementById('canvas2');       // layer 2
var canvas3 = document.getElementById('canvas3');       // layer 3

ctx.drawImage(canvas1, 0, 0);    // draw canvas 1 into main canvas
ctx.drawImage(canvas2, 0, 0);    // draw canvas 2 into main canvas
ctx.drawImage(canvas3, 0, 0);    // draw canvas 3 into main canvas

var dataUri = canvasMain.toDataURL(); //extract main canvas as url