How many days do you have in Italy? What time of year?

I like all 3 cities, but I think Venice is very romantic and much more relaxing. I went for my third time last year with my boyfriend, and we had a fabulous time -- gondola ride at dusk, special dinners, listening to the orchestras in Piazza San Marco every night after dinner with a bit of dancing thrown in.

I would choose Venice with Rome as an alternative. Rome is large enough to get away from the tourist crowds if one chooses. I do not have the same feeling about Florence.

An alternative is traveling through Tuscany & Umbria for a week or ten days. I have fond memories of Siena and Orvieto.

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It would be helpful to know the rest of your itinerary to understand your "winding down" comment. Also, what time of year because IMO Venice is not as romantic in July/August as it would be at other times.

And apparently you can fly out of anywhere in Italy...?

We are going for a total of 3 weeks. Hopefully in September so that we can do Oktoberfest in Germany We have not decided how many days we will spend in each country. This is our itinerary so far:
Oslo, Norway (staying for free with a buddy, so we will let him show us around).
Amsterdam- Heineken factory tour, Jewish Historical Museum, Zaanse Schans, Vondel park and Begijnof tour.
Munich, Germany- Dauchau tour, Newschwanstein and Linderhof castle tour, Oktoberfest, The Black Forest, and Burggarten.
Italy is our last stop, either Venice or Florence and Rome.

Also, in between all the guided tours and activities, we'd like to do our own wandering around and discover the cities.

Obviously, you have to decide how much time you're going to spend in Oslo, Amsterdam and Munich before you can determine whether there will be time for Florence and/or Rome after Venice. That's your call based on how you want to balance your days between active sightseeing and leisurely exploring. Personally, I think you're trying to cover a lot of geography in three weeks.

Unless you plan to fly between destinations, you're going to lose two entire days getting from Amsterdam to Munich and Munich to Venice, and you'll lose half days getting from Oslo to Amsterdam and Venice to either Florence or Rome. If you go to both Florence and Rome, you'll lose even more time.

I suggest rather than randomly assigning a number of days in each place that you create a timeline of what you want to see everywhere, how much time you calculate it will take to see those things, how long it will take to travel between destinations, and then figure out how much time is left (if any) for Florence and/or Rome.

BTW, there are countless romantic, honeymoon-worthy places in Italy outside of Venice, Florence and Rome where you would find loads of culture, art and unbelievably good food and wine. Your itinerary so far is heavily weighted to cities. I love cities too, but after 10 days or so I really crave more wide-open spaces.