Paris Pass

Generally the only one that is worth getting is the Paris Museum Pass. You'll pay more for the others than if you'd paid separately. The Paris Museum Pass is either for 2 or 4 days. Unless you are going to rush around for 3 full days doing only what's included (a few major sights aren't) and not do anything else, like a walking tour, a Seine cruise, coffee at a sidewalk cafe, a leisurely picnic lunch or dinner, only consider the 2-day pass. Whether the 2-day pass is good for you depends on the time of year (lots of tourists or not) and which sights you are interested in. The pass will allow you to skip lines at some places (Orsay, Versailles) that typically have long lines, but it won't allow you to skip at others where lines are long (St. Chapelle, Notre Dame Towers). And many places don't have long lines. It also depends on the days of the week. The 2 days are consecutive, so you need to plan how to use it wisely.

As Chani says, the Paris Pass is not considered a value.. but the Paris Museum pass can work out well if you plan on seeing at least 2-3 sites a day..
For transport for three days I would just get a carnet of tickets,, which is simply ten one way tickets sold at a discount to buying them singly.. They are good for bus or metro..if you are staying somewhere central between walking and uses a few metro rides that would do me for three days.. if staying out a bit you may need to buy and split one more carnet ( anyone can use the tickets) .. A carnet is about 13 euros.

"Is there a disadvantage to buying the "Paris Pass"?" The disadvantage of the Paris Pass is that you will waste a lot of money. They will dangle a long list of "covered attractions," but there are still only 24 hours in a day, and the attractions are only open a fraction of that time. Sure, the website boasts of over 70 covered attractions and activities, but how many can you actually do in 3 days? Follow the advice in the posts above. Forget the Paris Pass and Paris Visite. For a 3 day visit, consider a Museum Pass, but don't get it if you won't be visiting enough covered attractions to make it worthwhile. Here's a list of what's covered: http://en.parismuseumpass.com/rub-museums-monuments-by-name-5.htm. For transportation for 3 days, just get a carnet (pronounced car-NAY) of tickets, which is 10 tickets bought at once for much less than the price of 10 single tickets. When you need more, just buy another carnet. These tickets don't expire, so you can save any unused tickets for a future trip or give them to others. Note that tickets from the carnet won't be valid to Versailles or the airports; buy separate tickets for these.