Translation:We have slightly more than three weeks.
I think that might be because dryg also features with another meaning, so Duolingo thinks it's more common. It could also be that you've made a few more errors on such sentences so that the system has decided you need to practice those more. Or any of a few other reasons, hard to say for sure.
The problem is that I start second guessing myself. Now I don't know whether I'd actually ever say it or not - although I think it's fine grammatically. Is på oss necessary in the Swedish sentence? Anders91 says above that it implies some sort of deadline. Is that not true without it? If it is, then I would be inclined NOT to report it, because it certainly isn't necessary in English.
I don't fully understand the use of "på oss" here, and can't find any sources about it. Is it another definition of "ha på sig" (to wear), or entirely separate from the verb? Is it valid with other subjects: "Jag har drygt tre veckor på mig"? Other comments suggest it has to do with deadlines, so I guess it always needs to follow a time expression.
to me (Canadian native English speaker), adding the "left" indicates the timespan has already begun and the three weeks refers to the remaining time.
But "We have just over (slightly more than) three weeks" on its own can also be used before the activity has even begun, just to indicate the amount of time allocated for the entire activity.
So adding the "left" to the translation adds a nuance that is not indicated in the original.