Looking at visiting Hamburg in August. Should I allow 3 days to visit? It seems interesting.
Minimum in my opinion. It all depends on your interests.
If you have 3 full days devoted to Hamburg, or be still even more, then spend it there.
Several historical sites , museums, etc should that be your interest, in the greater Hamburg area. For the general overview, I would suggest a guided bus tour or the harbour tour at Landungsbrücken.
Clear yes. Hamburg has enough sights plus near interesting cities such as Lübeck, Schwerin and Bremen.
If you have a sunny warm day also a 1-day boat trip to isle of Helgoland is a nice option.
ABSOLUTELY. Best city ever.
This was the reason I went to Hamburg:
I just posted a bit in the other thread (10 days in Germany) about Hamburg and have posted a few other more detailed overviews in the last few years. August is a great time to visit, and three days will feel short. We aren't shy here about calling ourselves the "schönste Stadt der Welt", and most of us actually believe it.
If you have specific questions or want suggestions based on interests, feel free to reach out.