The third key in Egg Quaters is a tough one to get, first off you have to be in the green section of the quaters. The key is in the main area, but to do it you need to do some careful walking. First you need to put the treasure scope on and head down one of the hallways. You need to get a bomb from one of the Kikis and carry it back into the main green chamber. The scope needs to be on while you're carrying the bomb. Once in the main chamber you'll see a spring, it's only visible with the scope on, bounce onto it nad you'll be launched to a platform. Set the bomb down and get away while it explodes. Then get the third gate key.