Case power led needs three prongs, motherboard only has two

This situation happens frequently. Use a paper clip to force out one of the led wires, and move it over one position to create a space between both wires in the plastic holder. Check the white id markings or the board manual to see which wire to move. It's not a big deal if you can't get it working. You may break part of the plastic holder, but you usually have enough plastic left to hold the wire in place.