I use the Extang on my 150 Mike and it's excellent.
Has the 3 folds your talking about, is very strong and pretty well water proof and when the tailgate is locked, is quite secure , but we all know locks only keep out the honest folks.
I can remove it myself very easily when required and reinstall it in about 5 minutes.
It can be folded up towards the rear window when you want to carry bigger items which is nice.
I worked the price of it into the price of my truck when I bought it, but I think the value is around $900 if your purchase it seperately and I think their available from Van & Truck World.
You can buy cheaper ones, but you get what you pay for IMHO
It's an item I'd definetely recommend.
http://extang.com/docs/solidfold.html