SHOCKING new revelations in the case of a Michigan man whose three sons have vanished filled a courtroom yesterday. Among them: dreams of the three missing boys near a dumpster; the trio in blankets clutching their toys as their dad drove away; his chilling search for the best way to break a neck. John Skelton has pleaded no contest in the disappearance of his three boys: Andrew, 9, Alexander, 7, and Tanner, 5. The boys vanished last Thanksgiving. He has still given no explanation as 