The DX ringtones may be in a different location, but this is how I got to mine on the Incredible. I'm sure it's the same basic idea.
Using Astro file manager (not sure if the DX has its own built-in file manager)...
- Click "up" until you can't go up anymore and look for the "system" folder and open it.
- Look for the "media" folder and open that.
- Open the "audio" folder to see the folders for alarms, notifications, etc.
- Do what your file manager does to copy the folders and paste them to your (DX's) external sd card (or any directory that you can easily find).
- Transfer the sound files you put on the sd card to folders on the S3. I just had both of my phones hooked up on my computer and copied files between the two. I don't know if it matters where, but I just copied the sound files into "ringtone" and "alarm" folders that I made on the S3's internal memory. The sounds should show up right alongside the crappy ones.
It's probably easier to use Zedge if it has the sounds you're looking for, but I didn't know about it at the time. This is really just a simple copy and paste job, though. I just have a round about way of explaining things!