Firstly, an RPM file is a binary-only file... it contains the program and (some) files required by that program. An SRPM is the same as a .SRC.RPM... it contains source files, of a sort. If you're not interested in recompiling them for your system (not always necessary), then don't go for the SRPM files. As for installing the Mandrake specific KDE3 RPM files, there should have been a (text) file explaining what to do. If I remember correctly, you'll need to move to the directory that you saved the files into, su root to become the root user (temporarily) and then give the command urpmi *...
Don't take my word for it, though... I haven't done this in quite a while.