@BarbieQPickle,
Roberta wrote: But the question wasn't about 'a' or 'an'. It was about using a singular word (an) with a plural (three weeks).
Yes, Roberta is correct. It is not about ‘an’ or ‘a’, but about singular word used with a plural.
Barbie wrote: That is true you can use "an" before any word that starts with a vowel. If it doesn't, then you use an "a".
I think JTT is correct: That is true you can use "an" before any word that starts with a vowel sound. If it doesn't then you use an "a".
Not just vowel but vowel sound. For example, it is a university, not an university.