"has been in France for three weeks" does not at all mean the same thing as "went to France three weeks ago." The former refers to something continuous and continuing. The latter refers to a single event. "She went to France three weeks ago" could be true even if she was there for only a fraction of a second three weeks ago. "She went to France for three weeks" is closer to "has been in France for three weeks", but still isn't the same thing, first because the former doesn't tell us when she went to France or that she's still there and second because it limits her stay in France to three weeks. "She spent the past three weeks in France" would be better. But that's using "spent" not "went".
There's something else going on in this sentence. If you're expecting someone to return, and you know how long the person will be away, then "has been" tells you how much longer. If you're expecting someone to return, but you don't know when, then "has been" suggests uncertainty about how much longer. If you're wondering why you haven't seen someone around, then "has been in France for the last three weeks" is the reason.
I don't see any way to express the same meaning with "she went" and without "has been". If this is for an English course and you have an instructor who knows how to do this, I'd love to see your instructor's answer. Of course, the point may be for you to learn that it isn't possible. We have multiple tenses for good reason.