Can I download Photoshop 3?
I don't think so. I also don't think you can install it on any modern OS; Photoshop 3 was released before Windows 95!
Who/what is telling you to use Photoshop 3?
Well, you should write off those files. I don't know a single app that can open this old Photoshop format nor will it be in any way possible to install such old software on any recently modern computer, assuming you could actually find the program sources.
What operating system are you using?
What adobe software are you trying to open the pdd files with?
You should be able to open pdd files in any recent version of photoshop or photoshop elements if they are from Adobe PhotoDeluxe.
Which version of PhotoDeluxe did you save the files with and operating system?
Did you try File>Open As>PSD?
Moving this discussion to the Photoshop General Discussion forum.