Depends on which BT3 version you are using. Source is included in the USB version, not in the Live CD version.
Source files are in: /usr/src/linux-126.96.36.199/...
Not sure what iptables file you need to patch but iptables.h is in the /usr/src/linux-188.8.131.52/include/config/ip/nf/ directory.
If you need source try: http://www.offensive-security.com/kernel.lzm
To install it: lzm2dir kernel.lzm /