WE will be in Vienna,Prague and Budapest for 3 days this August. We were wondering if the cards are worthwhile if we plan on hitting the major sights and museums. Besides a discount, do they allow you to skip the line they way the card does in Paris? Also is it better to buy online here or wait till we get to each country?

ttt

just wait until you arrive. In Vienna, you can usually get one at your hotel. Greatest value is as a transit pass to allow you tram/metro access without having to buy individual tickets.

We were in Budapest last year and our hotel provided us with a card free of charge as part of the 3 day package. The only trouble is that one of the days - the one we had most relied on for sightseeing - was a Monday and almost everything (except) shops is closed in Budapest on a Monday. We were especially disappointed that we couldn't visit the Parliament building which looks so magnificent.
However, we still enjoyed our stay.

I forgot to mention that the metro system is very small in Budapest but the card does allow you to travel as much as you like and also on the trams.