TL;DR - The third thing was Sugar. Not mustard, not paprika, not dried herbs, not something lost in the mists of time.
It was sugar, and there's historical proof.
ETA: I'd put about 70% of this post together before @dduane said "Have you seen this?"
"This" was from @jesters-armed, in first with my notions about The Fifth Element Third Condiment, and even a mention that the posts were "...a bit long(ish)".
Yes they were, with no change here. You have been warned. :->
Well, okay, there's one change. The pix in this post are new and, combined with the illustrations in older posts, go even further towards confirming that what I once called a theory, I now regard as Fact.
Here are a couple of 19th-century table caddies, proper name "cruet sets". Take a look at the labels. They answer the "what was it?" question asked by that TikTok in a single word.
Not just in English, Spanish too.
Even without labels to tell them apart and even when the containers were of matched size and shape, sugar-casters always had larger holes than pepper-shakers.
Sometimes not much larger, as here...
...but usually, like those below and above, more than big enough to ensure no confusion between sugar and pepper.
A container of similar shape with no holes, as in the set above, held mustard.
Mustard was never a shaker seasoning; it didn't work that way. Its spiciness doesn't activate until the dry "mustard flour" was mixed with water, vinegar, beer or wine and left to stand for several minutes.
This produced a runny-to-stiff paste which was at first transferred from pot to plate on the point of a knife, but soon got its own dedicated spoon.
There's a slot in this mustard-pot's side for a spoon, and the set pictured above may also have such a slot, unfortunately facing away from the camera.
A matched spoon became part of any mustard-pot set...
...and was such a uniform size that "mustard-spoon" was a recipe measurement along with dessert-spoon, tea-spoon, salt-spoon and even cayenne-spoon. (I've posted about cayenne as a table condiment elsewhere).
Where's the salt-shaker in those sets?
When sets like those were in common use, salt-shakers weren't.
So how did people use salt if it wasn't in a shaker?
In the Middle Ages and Renaissance salt was put out in ornate dishes called a Salt which were often spectacular works of art.
This was placed at the top end of the table where important people sat; those seated further down were "below the salt".
Later, and still nowadays in formal settings, salt went into smaller dishes - salt-cellars - which like mustard had their own spoons. These were set on the table between two or four guests.
They took salt with the spoon, and instead of sprinkling it all over, they made a little heap of salt on the side of their plate and added pinches as required with finger and thumb.
The same side-of-plate thing is done with mustard.
English mustard is extremely pungent *, far more so than the Grey Poupon which TikTok Guy slurps so casually off his finger. A little can go a long way, too much can be overpowering, and slathering it over an entire plateful of food can make that food inedible.
(* I'm aware Chinese and Russian mustards are even hotter; they're not relevant here.)
I once had the educational (okay, also entertaining) experience of watching a friend from the USA putting Colman's English on their hot-dog as if it was French's Yellow, then taking a bite. Even then they were lucky, because mustard is hottest when made fresh and the shop-bought from a jar was much weaker than it might have been.
"Made mustard" of the kind which went onto Regency, Victorian and Edwardian tables packs quite a punch, and dishes of that period was far from bland; it took two world wars and their associated rationing to give British food its rep for being dull.
Here's an example of how mustard is used.
Even though it's from a jar and feeble by comparison with fresh-made, it's likely that most of this will remain untouched when the meal is over.
Jeremiah Colman, founder of Britain's best-known mustard company, was only half-joking when he claimed that the firm's excellent sales record, and his own fortune, came from not from mustard eaten but from what was left on plates.
Whether on the plate or on the food, mustard for table use never came out of a shaker.
The TikTok cites Bill Bryson, an American writer who, though living in the UK and presumably familiar with local grocery shops, failed to connect the proper name of the shaker ("caster" - TikTok Guy uses the name himself) with a grade of sugar sold by Irish / UK shops right now.
Here are the three standard grades - coarse, medium and fine. Note what the middle grade is called.
""Caster" has become a single-word description for "fine-grain quick-melting fast-mixing general-purpose cooking-and-baking sugar" but is a literal description both of how it was used ("cast" as a verb) and the container ("caster") it was in.
TikTok Guy mentions the "expense and effort" of using sugar.
From the Middle Ages up to the early 1600s sugar was indeed expensive and only for the rich.
Good Queen Bess's teeth were in an appalling state because of her sugar consumption, and less-wealthy people sometimes blackened their (healthy) teeth, to suggest they too could afford enough sugar to cause rich-people tooth decay.
However, increased use of slave labour on sugar plantations meant the end product became more and more affordable, and by the mid-1700s sugar was no longer "a luxurious delicacy". It became a household staple, enough that in 1833 politician William Cobbett ranted about how overindulgence in sugary tea had sapped the vitality of the English working class.
His remedy was home-brewed beer, and lots of it (!)
TikTok Guy uses the word as if it's something out of the ordinary, and seems unaware of how much physical labour - from preparing and cooking food to fetching water to washing dishes to tending the fire or range - went on every single day in a pre-modern-gadgets kitchen.
For instance, before electrical ease or hand-cranked convenience, whipping cream to thickness or beating egg-whites stiff enough for meringues meant thrashing away with a bundle of twigs "until it be enough", however long that took.
By comparison, breaking down a sugar-loaf was quick and easy, especially since there was a tool for the purpose called "sugar nips".
There's a set in one of the TikTok photos, though TikTok Guy didn't comment on them. He may not have known what they were.
Once nipped off, sugar chunks were reduced to the required texture with a pestle-and-mortar, exactly as was done with every other crushable ingredient in that period kitchen.
This and everything else wasn't effort in the way TikTok Guy thinks; it was just - especially if a mortar was involved - The Daily Grind.
I've posted about sugar casters before, and the first time (six years ago) was amusingly cautious:
So that third container was IMO for sugar.
Since then, backed with increasing amounts of hard visual proof as shown here and elsewhere, I've gone from caution to Certainty.
The "mystery" third container in table cruets was for SUGAR, with enough historical evidence in the form of specifically labelled and shaped containers to confirm it beyond doubt.
And they all sprinkled happily ever after.
