Despite the fact that Dealey Plaza was an “echo chamber,” McLaren’s book shows how some witnesses who headed for the Book Depository changed course after the third shot, which seemed to them to come from a different direction (i.e., “towards the grassy knoll”). One witness even said that the third shot sounded like it was coming “from right there” (i.e., from inside the presidential limousine).
The majority of witnesses reported hearing three shots, although some reported hearing as many as six. Witnesses also gave different origins of the sounds, like the ones who started towards the Book Depository after the first two shots were fired, then changed direction towards the grassy knoll after the last shot was fired. Even if they couldn’t specifically locate the exact source of the sound, they knew that the third shot originated from other than the Texas School Book Depository, and the follow-up car happened to be between the witnesses and the grassy knoll. The timing of the last two shots being "close together" and "almost simultaneous" was pointed out by McLaren as evidence that they could not both have come from Oswald's Carcano rifle.
You can see and hear some of the original witnesses describing the sounds they heard on this YouTube video: https://www.youtube.com/watch?v=ebvDHPk05FQ Although the YouTube poster believes that there were four shots fired (not three), the fact that the last two shots were so close together points to the accidental shooting by Hickey.
The failure of witnesses to agree on the number of shots fired in Dealey Plaza and to agree on the origins of those shots is due to the echoes created in Dealey Plaza by the buildings and especially by the triple overpass. Menninger’s and McLaren’s books point out the difficulties of analyzing ear-witness accounts. What the witnesses did hear, however, is consistent with Donahue’s theory. McLaren’s book is especially useful in pointing that out.