Maybe that third one doesn't have a membrane? Sometimes they are mysteriously missing. Seriously. If it's there but you can't get it off, just leave it alone. You probably won't notice a huge difference, just the meat near the backside of the bone will eat a litte tougher.
Some people score the membrane with a sharp knife in a diamond pattern instead of removing it, but I don't see how this does anything.
Regards,
Chris