Where [if anywhere] should the apostrophe go in "three days work"?
"Three days' work" is correct.
Yes, it would be different if it was one day: "one day's work" refers to a single day, so day would not be plural to start with.
It's "three days' work" because of the fact that there are three days. As this is a plural, the apostrophe comes after "s" to distinguish it from a singular, which has the apostrophe come before the "s":
The cat's fur/ singular
The cats' meows/ plural