"But when it comes to sightseeing what are the must sees?!!! This is where I am struggling. I get mixed review about everything. We are spending 3 days each in each city. Can I get some recommendations on what is worth the time and money ?"
With only 3 days, you will have to be very selective, particularly in Berlin and Vienna. But, as you say, everyone will have different views on the "must sees" and "must avoids" of each place. The best you can do is for you and your boyfriend to look at the lists of top attractions, learn more about them, and decide what draws you most.
If you want to see Rick's opinion on this, scroll down on each city's link and click on "At A Glance" for:
and Berlin https://www.ricksteves.com/europe/germany/berlin
In each entry, if you scroll down further under "Watch," you can see Rick's videos and travel talks. Just be aware that some of the videos are getting old (10 years or more).
In the end, only you can decide what's worth the time and money. For instance, I loved the German History Museum in Berlin and could spend a whole day there; others would be bored out of their minds in 30 minutes. The Pergamon Museum in Berlin is often cited as an absolute, ten out of ten, spectacular sight, but to me it was only "good."
And accept, right now, that even with the most intensive research and planning, you will not know how you will react to a sight or attraction until you've seen it. In Berlin, I only went to the Aquarium because my mother wanted to, but we both ended up loving it; I preferred it to the more famous zoo. The Berlin Botanic Garden, on the other hand, was distinct let-down for both of us.