How to set the intensity of the first graphics card 3 and the second 6?
No way. As a workaround for this issue, you might use a few CryptoDredge instances with -d option.
e.g.:
CryptoDredge -a -o stratum+tcp:// -u -p -d 0 -i 3
CryptoDredge -a -o stratum+tcp:// -u -p -d 1