Backing up a bit...
As an example, I saw this tour for Venice...
Your itinerary has you in Venice for 1 night, coming from Zurich. As you'll kill 1/2 a day or more just dealing with transport to get to Venice, this leaves you little time for the city. Consider that daylight will be short in January! Additionally, most of us do not advise booking tours for an arrival day should a transport or other delay cause you to miss the meet time. The tour you're looking at starts at 9:00 AM; there's no way you'd make that if transiting from Zurich.
Truthfully, I'd look at skipping Venice altogether and adding that night to Rome. There is a great deal to see there, and I personally consider 4 nights/3.5 days the minimum just to scratch the surface. Again, consider that daylight will be short!
Florence is very easily done on one's own, The historic center is not overly large, is easily walked, and you can use any number of self-guided walking tours, such as Rick's, to get your bearings. Depending on the sorts of things you wish to see there, you would want to pre-book entry tickets for a couple of the most-visited, such as the Uffizi and/or make the mandatory reservations for the duomo's dome, There is also a tourist pass available that may or may not be advantageous depending on the math.
Same for Rome. There are a couple of attractions you'll want to want to pre-reserve tickets for, such as the Colosseum and Vatican Museums, to skip long lines. That said, if you wish to visit the Palatine and Forum as well as the Colosseum (general-entry tickets are inclusive of all three), a guided tour can be beneficial as the latter two excavations are complex and tough to understand what you're looking at without a bit of advance reading.
Tours of the Vatican Museums can also be a plus, especially those which get you into the Sistine early, before the mob, and which include the basilica. That particular combo allow direct, shortcut access into the church from the museums, thus avoiding a long walkaround outside and another security check.
Two oft-recommended companies on this forum are Walks of Italy and The Roman Guy. I've not used either, personally, but many fellow posters have and given them high marks.
Otherwise, Rome is not difficult to explore on one's own with a good map and maybe a self-guided walking tour or two in a guidebook. Advance reading is HIGHLY recommended for ANY city on your agenda so you get an idea of how they're laid out, the sorts of attractions they offer which do or don't appeal, and how to visit those places (hours/entry fees/items which are or are not allowed inside/etc.).
Not much different for Paris, although it's a bit more spread out than Florence or Rome. We had a week there, didn't book any tours at all and didn't have any issue figuring things out on our own: a map, a good guidebook and advance reading... There IS a pass, the Paris Museum Pass, which is excellent if you are interested in enough in what it covers and can do enough of those attractions to break even, or nearly so.
