My comment from meta.SO:
And finally, the style of badges should be the same on this, this, and this page.
At the point where I'd say page/pages, am I talking about the group of three pages, or the last one page?
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Consider an analogous example:
Here the conjunctive noun phrase "the white dog, the black dog, and the brown dog" is a plural phrase, evidenced by the plural verb "are".
The same thing goes for an example involving complex demonstratives like "this page". Consider:
Notice that the plural "are" sounds fine here, not "is". This suggests that "this page, this page, and this page" is a plural noun phrase.
An elided version "this [page], this [page], and this page" would also be plural. On an alternative analysis which holds that "this, this, and this page" comprises two simple demonstratives followed by a single complex demonstrative (compare "this, that, and the brown dog"), it is still plural.
Thus, anyway you slice it, it's plural.
That said, just because it's plural doesn't mean the final "page" gets an -s. In fact, adding the -s would be incorrect. Lots of plural noun phrases don't get the -s. For example, the phrase "John and Alice" is plural, and yet there is no -s. It is the same for "this, this, and this page."
The question asked is whether page or pages is correct, but the title asks 'plural or singular?': The two are different. The phrase can be seen as plural as a whole, because it could easily be replaced with 'these pages'.
But, hidden behind that, it's actually a sequence of three singular phrases:
... on this [page], [on] this [page], and [on] this page.
Using pages would be equally incorrect in any one of these.
By the way, the quote would only have made sense in a conversation where someone was pointing to some design samples or holding them up. More objective, written language makes the whole question disappear: 'the style of badges should be the same on pages 17, 21 and 24'.