A note about visiting these attractions on a Sunday? As of July 1, 2014 all Italian state museums will observe a free day on the first Sunday of every month. This would include the Forum, Palatine and Colosseum but curiously, Musei Capitolini doesn't seem to appear on the list of state museums in Rome. Odd, that.
Anyway, if your trip just happens to fall on a first Sunday, the Roma pass allows you to skip the waiting line but expect these attractions to be mobbed. Reference this previous thread:
https://community.ricksteves.com/travel-forum/italy/colosseum-free-sunday-skip-the-line
If your Sunday is during another week of the month, and decide against the Roma pass, you may also preorder skip the line tickets for Colosseum/Palatine/Forum here:
http://www.coopculture.it/en/colosseo-e-shop.cfm
A ticket does indeed cover all three attractions, and yes, you can fit them all plus the Capitoline museums into one day: we've done so. Just be in line first thing in the morning!