In three days, will you... (inversion?)

I can't see why the second one would be used (unless it's a question, of course). Could you provide the whole sentence?

MustAskIs it an inversion?

Yes. It is "subject-verb" inversion. But it doesn't make any sense as it stands.

CJ

I have a better sentence to play with:

You should see an improvement in three days.

Now if change the order:

In three days, [you should / should you] see an improvement.

I guess the "you should" alternative is the right one?

Thanks!

MustAskI guess the "you should" alternative is the right one?

Right.

I could see an inversion in something like a prophesy: You will receive a visitor in three days and within the week will you know well the importance of that visit!

Kind of literary and old sounding.

It doesn't have the inversion of a sentence that starts with a different type of adverbial phrase like "Around the rock ran the squirrel, followed by the cat, chased by the dog."

BarbaraPAI could see an inversion in something like a prophesy:

I find this quite a stretch with this sentence. If it is possible, I think it would have to be without the comma?

It would absolutely require the elimination of the comma and a prophesy-like context missing from the original. I was simply stating that the construction is not impossible in every context.

BarbaraPAYou will receive a visitor in three days and within the week will you know well the importance of that visit!

MustAskI guess the "you should" alternative is the right one?

Yes. Just because you invert one part of a sentence with another doesn't mean you invert the subject and verb too!

Subject-verb inversion occurs in direct questions and when an adverbial with negation or "only" starts the sentence.

He has seen it.
Has he seen it?