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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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    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. – uhoh Mar 13 '21 at 00:14
  • That's right... I remember seeing something about a spare Soujourner rover. I can't remember if it was displayed someplace, but I remember seeing something about it online. – Deko Revinio Mar 08 '24 at 21:01
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I read somewhere that Voyager 3 was to have been launched in November, 1979 and flyby Jupiter in April 1981 /May, 1981 approximately, then fly onto Uranus in 1986/87, and finally Neptune in 1990/91. One article had mentioned something like that, and i believe it was the August 1970 issue of National Geographic, and Planetary Encounters book by Robert M. Powers.

from the National Geographic, page 185:

One voyage, beginning in 1977, is planned to visit Jupiter, Saturn, and Pluto, making the trip in 8 1/2 years instead of the 40 it would take to go to Pluto alone if this game of celestial billiards were not played. The other Grand Tour, starting in 1979, aims for Jupiter, Uranus, and Neptune (diagram) diagram showing two grand tour trajectories; one in green leaves Earth Sept. 1977, at Jupiter Apr. 1981, at Uranus Jul. 1985, and passing Neptune Nov. 1988. The other in orange leaves Earth Nov. 1979, passes Jupiter Feb. 1979, at Saturn Sept. 1980, and at Pluto Mar. 1986

Erin Anne
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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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    Do you have a reference that supports your statement "NASA decided that the Jupiter/Saturn/Pluto route was not important enough."? – Organic Marble Mar 11 '21 at 17:03
  • 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. – Saturn 5 Mar 11 '21 at 19:35
  • Somebody asked that question, that is why I answered it – Saturn 5 Mar 11 '21 at 19:42
  • Yes. I do not get what the disagreement is. – Saturn 5 Mar 11 '21 at 19:43
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    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. – Organic Marble Mar 11 '21 at 19:44
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    The other answer has no reference either. – Saturn 5 Mar 12 '21 at 16:46
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    @OrganicMarble see comment. – uhoh Mar 13 '21 at 00:16
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    @Saturn5 Welcome to Stack Exchange! I do see that the other answer also cites no sources and I've left a comment there. Nonetheless we should all strive as much as possible to cite sources to back up assertions we make in answers. Do you think that you can try to do so here? Thanks! – uhoh Mar 13 '21 at 00:18