Tour Companies

Among others:
Walks of Italy
Context Travel Tours
Dark Rome Tours (now part of City Wonders, it seems)

I believe all of those have various day-long, part-day or sometimes evening tours in all three of the cities you mention.
By the way, I understand that Viatour does not operate its own tours, but just books places on other companies tours. I don't know if Viatour prices are any more or less than if you booked with the tour company itself.

I second Larry's recommendation for Context. The tours aren't cheap, but they are small groups and your docent will be very well educated in the subject area. We took two Context tours in Amsterdam and enjoyed every minute. Euros very, very well spent!

I plan to book something with Walks of Italy while I'm in Italy this winter (their blog is also a very helpful travel resource), but unfortunately can't offer any help at the moment. Something about Viator always seems off to me, so I've been hesitant to book. I'm curious to hear what kind of experiences others have had with them.

We also found a fantastic history tour by sorting TA recommendations by activity/tour. For example, you can view a bunch of different day tours in Venice by clicking here.

In 2012, we used Rome in Limo for full day private tours in Rome and Florence/Pisa - we were on a cruise so only had one day in each place and Rome in Limo was awesome. I'm not sure if they do group tours (I didn't look into that). They had drivers/guides that spoke very good English and even arranged for us to have a specialized guide at the Vatican Museum in Rome (which was incredibly helpful).

We were also in Venice for 2 days before our cruise and used Karen with The Venice Experience for 2 private tours - one a 4 hour walking tour of Venice and the next day a 6-ish hour tour of the surrounding islands (Murano, Burano and Torcello). Karen is an ex pat living in Venice, originally from Maryland and she was wonderful. It wasn't particularly cheap, but again it was a private tour and she was extremely knowledgable and helpful.

I'd highly recommend both of these companies and am happy to provide additional information if you'd like.

Thanks for the replies, it does appear Viatour is a booking agent, however the reviews are stellar. The Last Supper Tour in Milan under it's own web site says "Tours booked by City Wonders" When you read the itineraries, they seem like very similar tours. Sometimes, just getting the right guide will make or break a tour.

In late Oct 2013, I took the Dark Rome (now known as City Wonders) Milano Tour on a Sunday afternoon. About 3 hours walking tour, starting with the interior and exterior of the Duomo (NOT including the roof), through the Galleria, walking by La Scala and through the Fort (but not inside), and then on to the church where the Last Supper is located, including the 15-minute visit to the painting. Nice guide and tour, but not great. Covered a lot of territory for the time, and it did include entry to The Last Supper, which is often hard to book on an individual basis, it seems. Nice painting, looks much like the photos, only bigger. (:->)))

I used Walks of Italy in Florence and Rome and loved them. We had a great guide and the groups were small about 20 folks.

I used Viator in Milan, Venice and Rome.
I was very pleased with the tours and the guides. As mentioned above - they are a tour
consolidator.
I found their prices to be very reasonable too.

I also signed up for this free tour while in Venice. The free tour reviews can be very mixed (lots of docent no-shows seems to be the #1 complaint), but the Venice operation looks good. We took a free tour in NYC and it was superb.

I used Viatour for tours in Rome and Paris. In Rome it was a night walking tour and in Paris it was an night tour of the Eiffel Tower and another a day walking tour. The cost was very reasonable. I believe no tour was more than $60. I also arranged a tour of the Louvre in Paris.