It is oddly phrased. So I think a pragmatic understanding is best.
There is some kind of rule. If the rule is "you can fill in the form during any 72 hour period"... what is the point of a rule. If you can fill the form in 10 days before, why even have a rule?
So it must mean in the 72 hours before arriving in Germany. Probably this is a form to confirm that you have passed a covid test or something similar that needs to be done "shortly" before you enter the country.
So what it "exactly means" is unclear (it is unidiomatic, and borderline grammatical) What it really means is pretty certain.