How to get the nuts/screws? out

I don't have the equipment at this time to take and post a picture. I figure if anyone knows what I'm talking about, they will be able to help me anyway. I need to take the gas filler tube out of my 68 Barracuda. But, unlike my other Mopars, this one does not have Phillips head screws. Instead, it has three screws with a sort of round cone shaped head, flat on top and, instead of a standard slot or Phillips slots there are 3 indentations at the edge of the nut. This is a California car and I suspect this is related to their pollution and no tampering laws (I lived there 10 years). I have tried vice grips, needle nose pliers, standard pliers, you name it but no luck. It appears these heads were designed for some type of device to fit over the nut, have nubs to fit in the indentations, and then turn them out. Pain in the you know what. I have fussed with this for many weeks. Getting the tank out was easier than these screws. Anyone know what they are and how to get them out?
Thanks