No they don't need a ramp. Generally they choose to use the same box for whatever reason (maybe easier to defend if necessary) but 3 boxes for 5 chickens would be plenty. They need to feel safe and secure. Generally a lip of about 3 inches (so the eggs don't roll out) however some folks turn a 5 gal pail on its side and use that for a nesting box. I've also heard of milk crates turned on their sides for the same purpose.
I have two 5 gallon buckets as my nesting boxes. You can see the picture in the photo gallery under chicken tractors. I have used milk crates before, too. I don't have a ramp in front of mine, but I did put a perch in front of them to make it easier.
i use milk crates but dont turn them sideways. they work great. i have 3 for 15 chickens because they all want to use the same one. some people use a drawer from chest of drawers or just any thing. you can nail 4 boards togather just make sure no nails are showing and it has a bottom