If you use the Knoppix 7.2.0 DVD you have already libgtk-3-dev. Look inside the list of packages at http://ftp.uni-kl.de/pub/linux/knopp...-l-dvd-720.txt

If you have got the Knoppix 7.2.0 CD you should find out the version of the package libgtk-3-bin which is also the version of the corresponding package libgtk-3-dev. If the appropriate version of libgtk-3-dev is no longer available in the repository you can search for it at http://snapshot.debian.org/ Download the package and install it using the program dpkg. If there are missing dependencies try
Code:
sudo apt-get -f install
first.

I hope that helps.