Why is 'an' used with 'three weeks'?

and a fair thee well to you too.

I didn't see Roberta had already answered the question.

The question wasn't "a" vs "an".

It was about the use of "an" with a plural noun.

When it becomes "amazing three hours" or "terrible three hours" then you can use "a" or "an" accordingly because it becomes the singular time period.

Certes, 'tis an auncient usage . . . will i well.

As recently as 200 to 250 years ago, there were still places in England (and perhaps in North America, although i doubt it) where it was commonly used, and even exclusively used.

There would have been no confusion in speech--"an" as an indefinite article is pronounced with an "a" such as in "can"; when it is used to mean "if," it is pronounced with an "a" as in "wan."

I can no longer think of any more ways to work "an" into this post, but i'll try again, an ye list.

(I believe you have misspelled "hight," although i am, very occasionally, wrong.)

Curses, ye knave!

Zounds, varlet, wot ye well I'll nae suffer sich maldison!

Egad. Ods bodkins.

It is gratifying to think that we descended immediately into the ridiculous without bothering to begin with the sublime.

Yup. We jumped directly to sub-blimey.