I would suggest you a simple way to do what you want -
Install a fresh mandrake system with the software you want to isntall (not more then that)... It will take you long time to select all the packages from the isntallation list, but it worth it. Then, after you finish the install and boot into your newly installed Mandrake - verify you have all the things you need. then in terminal:
$ rpm -qa > rpms_list.txt
This will create a file named 'rpms_list.txt' under the current location. This text file will contain the list of all the rpms currently installed on your system. Now you have to do some work... Go over all 3 cd's and gather all the rpms that noticed in that files in 1 directory. After you finish doing it - cd to that folder (in terminal) and run this:
$ genhdlist ./
This will create a fresh new hdlist.cz file under the current directory, that will hold all the rpms that are there. From now on - all you have to do is mount the 1st iso, copy the newly created folder with the rpms you want instead of the one that is there and handle the hdlist.cz (put it where you need to put it). About the isntallation - i don't really sure how to handle the installation process and make it show the packages that you've added there, and not the default package list that came with mandrake.
I hope this helped you with something.
Phazeman.