I think part of the confusion here is that many people believe that the "volcano" manhole was the manhole in question. That is not the current belief of many who believe in the MHT. There are other manholes in the area that are not as deep as the volcano manhole (which is, IIRC, about 10 to 12 feet deep). IMO, the manhole used was the one about 100 feet from the discovery ditch. I believe it's referred to as the "thicket" manhole. Please, some one correct me if I'm wrong. It's not that deep, and removing the bodies from it could easily be accomplished by one man.
IMO, the killer removed the bodies one at a time and at a time when he was sure that the woods were deserted (as he had been a participant in the search). After the first two bodies were removed, they were transported to the discovery ditch. Then, the third body and the clothes were transported. This scenario also explains why two of the bodies were found removed from the third body.
As to the swimming pool theory advanced by another poster, I'll admit that it seems to be a possibility. However, I'd think that a pool would be one of the first places searched by the police, especially an unused pool on the property of one of the missing boys. I'd think that the killer would fear that possibility, too. Again, since the water in the lungs was never tested (and is now lost), once more the Keystone Kops thwart justice!