You need to get underneath and see what you've got - what size and how much room. 3mm sounds a bit small. You may need to cut off an allen wrench. It seems to me like Sears sells some low profile allen wrenches of some sort - not sure about that. Or you could cut off an allen wrench, drive the piece into a nut and put the nut in a swivel head, racheting box wrench. You just need to improvise based on how much room you have.