What is the name of this screwhead?
It looks like a tire lug screw.  You could try a multi grip universal socket like the one below depending on the actual size of you screw. 



What I usually do in cases like this is get out my Dremel tool with a cutoff disk and grind a slot in the middle of the screw and then use a large screwdriver to undo it. When that hasn't worked, I pounded a copper pipe close in size into the screw and made a makeshift socket, flattened the pipe and used an adjustable wrench to turn it out.