Can French/British be used as plural nouns?

The words “British” and “French” can be

(1) nominalised adjectives (compare “The poor are revolting.” See https://en.wikipedia.org/wiki/Nominalized_adjective), which take a plural verb form or

(2) as common adjectives (a)predicatively: “He is French/British”, and (b) attributively: “They are French wines/British cheeses.”

Adjectives are not inflected, thus all nominalised adjectives act as uncountable nouns, thus a French and a British are not possible.

This differs from the words American and German, etc., in which the words American and German may act as

(1) pure nouns - "Americans/Germans like beer" and "An American/German was elected," or

(2) adjectives - "I like American/German beer" and "The beer was American/German"

But not as nominalised adjectives:

Compare

The poor like food.

A poor like food. is wrong

The/An American likes food. Here, American is clearly singular.

but

The/An American like food." is wrong.

No, French and British cannot be used as plural nouns. Generally, you cannot use an adjective of nationality as a plural noun if there is already a different noun for a single person of that nationality.

So for example, you cannot say:

Three Spanish were ...
Three Polish were ...
Three Turkish were ...,

because you would use the words Spaniards, Poles, and Turks, instead. You can, however, say

Three Japanese were ...
Three Portuguese were ...,

because there are no separate nouns for people of these nationalities. See Google Ngrams.

Historically, the noun for British people was Briton and for French people was Frenchman. Today, you could still use Briton, but you might want to use Brit instead, especially if you're speaking informally.

Unfortunately, Frenchman is not gender-neutral, and so probably should not be used today. It's possible that French will eventually replace Frenchmen as an acceptable plural noun, but it hasn't yet. The best current alternative may be

Three of the French were ...

which I don't particularly like, but which is much better than Frenchies, which is often derogatory.