I am going to Rome for three days (arrive Friday morning and leave Monday morning). Any advice on an itinerary? Thanks!

Hi
If you go elsewhere here at fodors.com to Destinations and click on Rome, you will find lots of sightseeing information, including itineraries for short visits.
I suggest you think about a walking tour agency that will help you hit the highlights. One popular one is Enjoy Rome, another is Through Eternity.
My choice is Scala Reale. They are pricer than some, but offer 4-hour tours in small groups of locations like the Vatican, and the Colosseum/Forum.
They also include a 2-hour Orientation Hike or Walk. Enjoy Rome tours are shorter. That might appeal to you as well.

You will need a good map and guidebook.
If you don't take guided tours, many of the biggies have acoustiguides.

The usual sightseeing highlights are
Vatican Museums and St Peters -one half to one whole day

Forum and Colosseum

Spanish Steps/Trevi Fountain/Piazza Navona (1/2 day, including browsing)

One major museum---choose from among Capitoline, Borghese Gallery, etc etc
(depends on your tastes)

You can choose also from literally hundreds of churches most of which are noted for their history, their architecture, and the famous works of art they contain

Get one of those detailed maps - they'll probably have a free one at your hotel - and go! You might consider one of those inexpensive Rome Vision bus tours to start, just to get oriented with city and figure out where the sights are. Almost all the biggies - Pantheon, Trevi Fountain, Victor Emannuele, Spanish Steps, Piazza Navona - are close to each other off the Via Corso, so you can do them all in a day. The Colosseum, Forum, Circus Maximus and other ruins are close to each other, so you might want to pack them and the Vatican into the next day. The Metro in Rome is pretty limited, but it can get you into the center quickly. From there, everything's close.

One of your two full days is a Sunday. Some sites may be closed or have limited visiting times. As you plan your itinerary, note opening times carefully. Churches, while open on Sundays, are having services and do not want visitors trooping through. We used Sunday as a day to visit Ancient Rome. The Colosseum, Palatine Hill, and Forum are all open.

The key is to read as much as you can in advance of your trip, so you're familiar with the layout of the city and get a sense of what you're most interested in seeing and doing (some people love Renaissance art, some love ancient history, some love shopping, some love just sitting in a cafe). The Frommer's guide to Rome includes a street map in the back that's quite good. Once you get there, you don't want to waste an hour every morning debating what to do and where to go and figuring out things like how to get tickets to the Vatican.

Then don't worry about seeing/doing everything, as there's no way you can. Make two lists: a short list of your must-see-and-do items and another list of the things you'd like to see and do IF you have time.

Finally, take time, especially on Sunday, simply to wander around and soak up the atmosphere!