Originally Posted by
Wombat1
...There is a shuttle from Terminal 3 to Terminal 5 for those who arrive at FCO on the train.
The shuttle bus location is poorly marked, IMHO. It's at the far end (left side, as you look at terminal) on the 2nd level. You'll see some concrete blocks just beyond which a couple of extra lanes cut off the main terminal road. Bus stops in one of those lanes.