This is the statement Ralph makes toward the end of chapter five after the night assembly when they missed being saved. Ralph had attempted to re-establish the importance of fire for rescue as well as other, civilized behavior. Simon makes an effort to explain that there really isn't a physical beast but just what's inside of them. Piggy attempts to agree, but both are verbally shot down quickly by insults from Jack. Jack then gets everyone excited about the possibility of hunting the beast and leads them all chanting and yelling down to the beach, leaving only these three by themselves. Because it is so dark, Ralph makes the comment "Fat lot of good we are. Three blind mice." This statement is most likely a combination of the fact they literally can't see each other right now, and also that they can't beat Jack right now. His ideas of hunting and having fun are too appealing to a group of young boys who want nothing to do with rules and responsibilites. These three are like mice in that they are too meek and weak to stand up to Jack and re-assert leadership.
Ralph, Piggy and Simon are coined the Three Blind Mice for several reasons. 1: Sing the song to yourself. "See how they run" These three boys do not like confrontation. They run from it. 2: The boys are oblivious as to what is going on around them amongst the rest of the plane wrecked crew. They are blind to the other characters and their motives.