Translation:The newspapers have written about three crimes the last five days.
Hi, I'm from England and certainly without "in" or something else to clarify after "crimes" doesn't sound right to me. You'd either need "for" (meaning that the papers have written about the crimes for 5 days, although the crimes could have been committed before that) or "in" (meaning that the 3 crimes have been committed during those 5 days. Even then it'd probably be better to add "committed" as well.) I hope this is helpful.
As a native English speaker, I definitely agree with the others in regard to the necessity of including "in" in "in the last five days". But what about the Danish sentence? I wrote "Aviserne har skrevet om tre forbrydelser i de sidste fem dage" (NOTE: "i de sidste fem dage"). Is this incorrect? Duo says it is wrong (should be no "i").
I take it all back. Maybe the original Danish sentence and Duo's English translation are fine. It simply means that three crimes occurred (at some point) and that the newspapers have written about them every day for the last five days. (Although if this is the case, then the English sentence might better be "FOR the last five days").