Rome in less than three days

It sounds like 2 days at best to me. First of all are you arriving on Mar 25 from the US? If so, you'll be dealing with jet lag sleepiness as well on Mar. 25. Assuming you are awake on March 25. I suggest you use the evening to visit things that don't close down for the evenings like museums and churches. For example, I would od MARCH 25: DAY (afternoon/evening) OF ARRIVAL I would walk around and visit: - Pantheon (first before closure), Spanish steps, Trevi Fountain, Piazza Navona, etc. all in the Centro storico area (ckeck). MARCH 26: - Next day I would devote to visit Ancient Rome sites: Forum, Colosseum, Arc of Constantine, Palatine, Caracalla Baths, Circus maximus (check).
Leave the evening to relaxation and walking around the centro storico for the things you couldn't see the day before. MARCH 27: - Vatican: Museum (short 2 hr itinerary) + Basilica di San Pietro. (check) I would leave Villa Borghese for another time, unless you can squeeze it in after the Ancient Rome or after the Vatican.

In terms of transportation around Rome the best way is to use your feet. The Vatican is a little farther and might require a longer walk (or metro or taxi or bus). From the Spanish step, you can take the Metro (Spagna station) and be near the Vatican museum (Musei vaticani station, or also Ottaviano, station) in minutes. I don't care much about the hop on and off in cities like Rome. Those tour buses can't enter half of the places due to large areas being closed to traffic. Except for the Vatican, which is along walk, but an easy subway ride away, everything else is within a reasonably short walking distance. Getting to Venice is faster by high speed train. 3 hrs and 50 min. from downtown Rome to Venice Santa Lucia station right on the Gran Canal. By plane, by the time you include transfers to airports (both far from center of town), check in and security, flight time, deplaning etc. you are looking at 4 or 5 hours at best

If the only thing you have planned for the 26th is the colosseum then you'll have a lot of time left in the day. As for the romapass, buy it when you get to Rome. There is no advantage to buying it online. I would book the Borghese as soon as the tickets become available. If using the romapass for entry you'll need to call to book the reservation. Just let them know you'll be using the pass for entry. You can book tickets or a tour of the Vatican museums online at the direct site. These will allow you to bypass the lines. To get from the Borghese area to the Vatican you can take the metro to the ottaviano stop. To get to the colosseum you can take the metro to the termini, switch lines to get to the colosseum. It takes about the same amount of time to fly to Venice as it does to take the train when you consider getting to the airport two hours early, the flight and then getting into Venice. I would just take the train. You can take a cab to the hotel for the fixed rate of 48 euros if it's within the city walls or you can hire a private company like romecabs.com to take you to your hotel. They cost about the same. You can also take the express train to the termini station and then take a cab to the hotel. There are lots of options. After a long flight I like to have a car waiting for me so I don't have to worry about catching a train or waiting for a cab. Donna

If the 25th is your first day in Europe then I would say definitely take the Hop on Hop off bus. You just might be a bit too tired to walk for miles. I took my grandson on the bus our first evening in Rome and we got a great overview of the city for the following day. It was not our first day, but it was summer and so hot... we needed a break to just relax. I was in Rome another time in Mar. and took the bus again...this time because it was cold, windy and drizzly. As far as getting from the airport... first trip had a $75 crazy taxi ride...second time I learned my lesson and stayed at a more central hotel that had a free shuttle from the airport. City buses are also available if you dont want the hoho bus.

Actually we find the hop on/off buses very good for orientation and a quick overview, and sometimes a little transportation. The ticket is good for 24 hrs so you can continue to use it on the 26th. You may have trouble staying a week on the evening of your arrival day.

The tram #19 is useful for going to the Vatican from that area (I live nearby), and also to the Colisseum (#3). Otherwise your feet or a bus are best as the Metro is not close. (You can connect to the Metro at the Colosseum or Vatican, though, and each is about 20 minutes away via tram.) That said, I think Roberto has laid out a pretty good itinerary for you to get a taste of this fine city. Don't overbook yourselves. Leave time to enjoy walking around. Pick the Vatican orthe Borghese Gallery, not both in such a short visit. Do reserve ASAP since you have a tight schedule. The train to Venice is actually more efficient than flying. You go from city center to city center. Look at both Trenitalia and Italotreno for options. There are discounts to be scored if you buy in advance.

What would be the best train to take to Venice; the Trentalia from Roma Termini or the Italoteno from Roma Tibutina? How would I get to the Tibutina from Parioli area?

I prefer the minibus #116 that has several stops in the Borghese Gardens and then fits into the smaller streets where regular buses can't go in the Centro Storico. If you buy a day-ticket or the RomaPass, it works like a hoho bus. Stops include Galleria Borghese, Pantheon, St. Peter's, Piazza Navona, Campo dei Fiori, Via Giulia, Via Veneto, Piazza Barberini - a nice orientation of the historic center. Remember to validate your bus ticket or show your RomaPass.