In researching the answer to this recent question, I came across some information that was new to me. A third Voyager mission was planned, and then canceled. Apparently, Voyager 3 was cannibalized during construction:
I am currently reading the book Voyager: Seeking Newer Worlds In The Third Great Age Of Discovery by Stephen J. Pyne. On the second chapter, it is listed that there were three Voyager spacecraft. The second Voyager, VGR 77-2 had flaws and it was used for spare parts for Voyager 1 (VGR 77-1) and Voyager 2 (VGR 77-3).
At one point, NASA had a Planetary Grand Tour plan that consisted of 4 missions (Mariner 11-14). Was Voyager 3 one of these spacecraft?