200 cigarettes is the limit, or one carton, that you can bring back in to the states duty free without having to pay tax. If I were you, I'd just toss them in your suitcase. They'll ask you about them when you come through customs. You have to fill out this form and declare the items that you're bringing back. They'll give everyone on the plane a form to fill out before you get off of the plane. I usually don't even put anything down when I'm bringing a couple of packs of smokes back, and they've never said anything to me, but reastically you should declare the three packs on the form to avoid any headaches.
i imagine that they might hassle you a little bit if you don't declare them, and then they find them in a subsequent search of your bags, so just mark them down to avoid the headache. I've been lucky enough to never have them really search my shit, so it's never been a problem for me, but it'd be a bitch to be held up in customs for something as stupid as not declaring an item that falls within the limits of what you can legally bring anyway. PM me if you have any more questions about the customs process.