Keep in mind that a Roma Pass is time sensitive. Your three day window of use starts the first time you have the card read at a site, this validates the card. How you have used the card at sites is all kept in a central data base. The bus and transit system seems to time stamp it in a different manner. Always ask if the site you are visiting has a Roma Pass discount, even if its not on a published list. The Roma pass is a network and not entirely tied to state managed properties.
Ostia Antica is a Roma Pass Site. And we got let in free, even after using up all the free entries as specified. We took the River Boat to Ostia down the Tiber. This is truly back door travel, you arrive at the back door to Ostia Antica. Take the tram back.
Its also good for unlimited use of the subways and the city buses.
The Window of Use needs some explanation though. It isn't 72 hrs. Its really 48 hrs. plus the remainder of the day, till 12am, of the day on which it was first used. So if you first validate (use) the card at say, 4pm, 4pm till 11:59pm counts as your first full day of use. To maximize the pass, get it early and use it early on your first day.
An inexplicable thing happened with my wife's card and my card, bought and used at the same time. My card stopped working on the third day as spelled out in the Lit. and as expected. My wife's card however, was good for an entire extra, "forth day." The only thing we did different was that I used mine a few more times on the Subway then she did.