The difference between Region 1 & Region 3 DVDs:
Region 1:
The quality of sound and visual data is the same with the ones in the original movie.
Usually have special features (like "Behind the scenes", or "trailers")
Usually have more different language tracks and subtitles.
Several DVDs come with special items. (For Example, a free ticket of "The Mummy Returns" when you buy The Mummy: Ultimate Edition)

Region 3:
The quality of sound and visual data is slightly worse than its region 1 counterparts.
Most dvds' special features are missing.
Some dvds may not contain the special items.


Above information comes from my experience of buying dvds.