Huan, the Hound of Valinor, was permitted to speak only three times - why?

Question

The great hound of Valinor, Huan, had many talents outside what is typical of hounds, one of which is speech. However, Huan was not allowed to just speak freely but instead was only able to speak thrice before he died.

... and Huan understood all that was said. For he comprehended the speech of all things with voice; but it was permitted to him thrice only ere his death to speak with words.

The Silmarillion, Of Beren and Lúthien

As far as I can tell, there is no explanation for this limitation on Huan's ability to speak.

My understanding is that in Tolkien's mythology magical ability and power tends to diminish incrementally over time and after use until the once powerful beings are simply no longer as powerful as they were in the past and can no longer do the same awesome acts (be it creating mountains, taking a physical form, building giant and impenetrable towers, etc.). To me, the fact that Huan is given a finite number of times to perform this magical activity is out-of-place in Tolkien's mythology.

• Is my reading correct that the Valar (or Eru) gave Huan the ability to only speak no more than three times? If so, why? I would be particularly interested in an out-of-universe reason as to why Tolkien chose to make this (as far as I can tell) unusual choice of giving Huan a specific number of occasions to speak, though an in-universe would be interesting as well.
• Or, am I reading this incorrectly and this was actually intended to be a prophesy (or doom) wherein when Huan speaks for the third time his death will soon follow (e.g. Huan was technically able to speak as much as he wanted but a prophesy indicated once he got to three he would have no more opportunity)?

Answer 1

According to the frequently-reliable Tolkiengateway, regarding Huan, there were separately one grant, and one prophecy, but they were about different things:

The grant:

Huan had been granted special powers by the Valar, he was as large as a small horse, immortal, tireless and sleepless, and was allowed to speak three times before he died.

The prophecy:

It was also prophesied that he could not be killed unless it was by the greatest wolf that ever lived; in this case a werewolf.

Sauron imagined himself to be just that, because he obviously must be The Greatest In All Ways. He wasn't. That was a separate matter, which Carcharoth politely took care of when the time came. Prophecy fulfilled.

To speak three times was a grant.

Edited to add 15 October 2021:

I am copying my answer from the comments, in response to OP SethMMorton's question asking if there are other instances of Tolkien specifying the mystic number three, because even if not out-of-universe, as OP says they may shed light on the question:

Here are some examples of the number 3 in Tolkien. There are 3 Silmarils, 3 types of Hobbit, 3 houses of Edain, the 3 hunters, 3 times Bilbo braves the secret passage ("third time pays for all" he says), 3 peaks of Thangorodrim, and 3 unions of Elves and Men. There are three children of Hurin, there were three Themes in the Great Music, and LotR is set in the Third Age.

Answer 2

I don't know of any explanation that Tolkien gave for why Huan was limited to speaking on only three occasions, neither in-universe nor outside. I do agree that it is a little unusual for his work. Some of the medieval romances feature this sort of arbitrary limit, and they seem to have been among the models for the story "Beren and Luthien", so it is possible that this feature came from such a source, but that is pure speculation.

The late comment suggesting that Huan was in fact a Maia is apparently very little developed. I gather that Tolkien sometimes jotted down ideas in this brief fashion. Sometimes he later developed them at length, sometimes he later explicitly rejected them, and sometimes he did neither. Unless there were additional, more developed mentions of this that I have missed (which is surely possible) I would not call this clearly canon. Other Maiar were embodied as men or elves, including Melian (wife of Thingol and mother of Luthien), and Gandalf and the other wizards, so there seems to have been no rule against doing so. Melian was even able to have children with an elf, and is an ancestress of both Elrond and Aragon.

And of course there are beings of unexplained origin, such as Tom Bombadil, who seems neither an ordinary Man nor a Maia. Where did Beorn's ability to change to bear-form come from? We are never told, AFAIK.

Te limit does add significance to each speech by Human, and perhaps that was the only reason.