Well, the possessive of “campus” is indeed “campus’s,” but you example is wrong because “campus” is not the plural of “campus” – it’s “campuses.” Therefore, the sentence should be “All three campuses’ dining halls serve Coke,” because the possessive of a plural ending in “s” is formed by adding an apostrophe. No, I don’t have a style guide handy, but I know damn well that “campus” is not plural, and so do you.
With plural possessives, style guides generally recommend adding the apostrophe alone to regular plurals in –(e)s, and ’s to irregular plurals (men, women, mice, children). Increasingly it is acceptable to leave our the possessive marker altogether in certain contexts (e.g. Teachers Union, in six years time). (It was difficult just to write those.) I know of no style guide that recommends adding 's for the pural possessive.
There is a lot more variation in what style guides have to say about the singular possessive, with both " James’ " and " James’s " accepted. Chicago even recommends “For conscience’ sake” to reflect the spoken pronunciation.
I can’t see why any style guide would accept that. In order to make a plural possessive, you have to make the noun plural (campuses) and then add the possessive (campuses’).
“Campus’s” is always a singular possessive.
The odd AP exception being that if a singular word that ends in “s” is made into a possessive and the following word begins with “s,” the possessive is indicated only by a single apostrophe. So: campus’s reputation, but campus’ safety.)
A plural possessive ending in -s (as noted, a few plurals are not formed with -s or -es) will never take -s’s but always -s’. (It’s my custom, in accord with some style guides, to form the possessive of the handful of borrowings from French whose plural is formed with an -x sounded as /z/ with a simple apostrophe, as though the -x were an -s. But opinions on this will differ.)
A singular possessive is governed by the style manual applicable to what you are writing. In th absence of style guide directives to the contrary, a good working rule is: Form a singular possessive with -'s, even with words ending in -s, except:
– Classical names like Achilles and Jesus normally take only the -’.
– For words in which the spoken singular possessive does not add an /-iz/ sound to th one already there, the -s following the apostrophe is discretionary and is commonly omitted.
I do think there’s a transatlantic divide here. The trailing possessive apostrophe on the end of a word that ends with S looks very weird to me.
My edition of Fowler’s says, bizarrely, that “s’s” is the correct form of possession for words ending in S - e.g. James’s post - except in the case that the last syllable begins with S too. The example given being “Jesus’ disciples”.
The last is an ad hoc rule created to explain the common practice of using the apostrophe alone after names derived from Greek and Latin sources, but not English, where the final letter of the singular form is –s. You can argue about the parameters of this: “Jesus” is clearly English by now, and “James” is ultimately from a Classical name. I seem to recall that Chicago’s version has to do with the number of syllables in the name.
To make it relevant to the OP: If Jesus were cloned and each clone had a lollipop, the collective candy would still be the Jesuses’ lollipops, and NOT the Jesus’s lollipops. If Augustus and his clones were to take them away, that would be either the Augustuses’ bad behaviour or the Augusti’s bad behaviour, I think; I’m pretty sure the phrase “bad behaviour Augustorum” would not be allowed. (No copy editor on earth has the power to stop a legion of Augusti on a sugar quest.)
Just listen to a bunch of old world preachers: they often say “in Jesus’ name” instead of “in Jesus’s name.”
So I think what they are trying to codify is what is already common practice in speech. The code I learned was one with a cutoff date for how long ago the person lived. Although I can’t remember the exact year, Jesus clearly fell on the s’ side of the line.