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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?

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Voyager 3 would have been Mariner 13, before the name of the mission was changed. It's actually fairly common when multiple spacecraft are designed to have one with flaws that is used primarily for spare parts. It is also common practice to build a spare spacecraft.

In any case, when there were 4 spacecraft there would have been two Jupiter-Saturn-Pluto missions, and 2 Jupiter-Uranus-Neptune missions. As these two were redundant, they canceled one of each. My guess is that at that time, they had the parts to build 3, and decided to build all 3 of the spacecraft. One of these had issues, and thus was never launched.

As for what the original purpose was, it is necessary to realize the primary purpose of the Voyager probes was to study Saturn and Jupiter. Voyager 2 received an extension after Voyager 1 made a close pass of Titan, and not before. Thus, there are 4 possibilities:

  1. Jupiter/ Saturn/ Pluto
  2. Jupiter/ Uranus/ Neptune
  3. Jupiter/Saturn (Only)
  4. A spare/ EDU system (Engineering Design Unit, used on the ground to verify commands and troubleshoot problems

If Voyager 3 had launched, I predict it would have followed the Jupiter/ Saturn/ Pluto approach.

As to why it was canceled, that is certainly part of the Planetary Grand Tour budget crisis that happened post-moon launches.

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    $\begingroup$ this comment illustrates why it's important to set a good example for new users and include sources in answers. Answers need to do more than demonstrate personal knowledge, they need to demonstrate where the information is coming from when possible. $\endgroup$ – uhoh Mar 13 at 0:14
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NASA decided that the Jupiter/Saturn/Pluto route was not important enough. They were low on budget of the moon missions (which costed 2 billion per launch). However New Horizons, launched in 2006, did do that route.

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    $\begingroup$ Do you have a reference that supports your statement "NASA decided that the Jupiter/Saturn/Pluto route was not important enough."? $\endgroup$ – Organic Marble Mar 11 at 17:03
  • $\begingroup$ It is obvious that that was the case. When they had to leave out one of the 3 routes, they chose that one, which shows that they decided it was not important enough. $\endgroup$ – Saturn 5 Mar 11 at 19:35
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    $\begingroup$ There's no disagreement. You simply need to back up your statements here with references, otherwise it's Some Internet Person Saying Something, which is worthless. $\endgroup$ – Organic Marble Mar 11 at 19:44
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    $\begingroup$ The other answer has no reference either. $\endgroup$ – Saturn 5 Mar 12 at 16:46
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    $\begingroup$ @OrganicMarble see comment. $\endgroup$ – uhoh Mar 13 at 0:16

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