I'm not familiar with how the Audition 3.0 noise reduction works but I've found basically 2 different solutions that seem to remedy the background hiss issue. You can mix in the hiss throughout the scene so that it's uniform over all the voices. Not something I would recommend unless you just have no other choice. Or put in an ambient background sound effect like wind or rain or perhaps music. Just something loud enough to help cover the hiss. A lot of times, things I stress out about in the voice tracks become minor issues when the rest of the sound is added to the final mix. Good luck!