That depends. What region is your PS3 from? If its from the US then US DVDs will work. If you are trying to play US DVDs on a European PS3 then, no, that won't work. You need to get DVDs from the same region as you got your PS3 (so if you live in Europe buy European DVDs, etc).
The PS3 is only region free when it comes to PS3 games. PS1 games, PS2 games, DVDs and most bluray movies are all region locked, and will not work across regions.
