I might be a little late to the party, but the 3 bag system I was issued last year came from a manufacturer here in Colorado. The company is Wiggy's and they are based in Grand Junction. Here is a link to their website.
Overbag (FTRSS)
The Flexible Temperature Range System (FTRSS) used have it's own product designation, but he has changed his website and now it looks like you choose your basic bag and then add the FTRSS option and an overbag/bivy.
I have used mine several times in the mountians during hunting seasons and the system is awesome. Thank God they don't issue us those old extreme cold weather bags anymore. I'm 6' 3" 215 lbs and I could never fit in the old ones. I fit in this one with ease.