Can French/British be used as plural nouns?

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.