Whatever you do, DO NOT, DO NOT, I REPEAT DO NOT smoke "just one". It never ends up being just one. I don't really know anything about herbal substitutes however, in my opinion you shouldn't smoke anything period, especially if you've already made it 3 months. Instead just keep busy and find alternate outlets for your craving like going for a jog, or writing a hub ;). Think of how much better your life is without cigarettes, feeling better, no ash tray smell etc... If any of this doesn't help, I suppose the next best thing would be to try one of those electronic water vapor cigarettes. I don't know first hand but they seem to have great reviews and appear to have a few benefits, no bad smoke since its water, multiple flavors, and can be smoked anywhere. The only issue I see with smoking anything is that it is too similar to cigarettes and it is much easier to justify since you are already smoking something else.
I know how hard it can be. I smoked marlboro reds for 9+ years and the reason I was able to quit was Army Basic Training. Four months of not having the option to smoke works wonders. There was a slip up or two along the way afterwords, but some perseverance and remembering why I wanted to quit helped. I still get the cravings occasionally, especially at the bar, but I just try to remove myself from the situation.
I hope this helps you and good luck to you.