Ronda Rousey is used to big challenges, and her role in “The Expendables 3″ is just that. The UFC women’s bantamweight champion is no bit part in the action flick; she’s one of the “Young Expendables,” according to a working draft of the script obtained Wednesday by MMAjunkie.com (www.mmajunkie.com). Rousey plays Luna, an “athletic female bartender” in her 20s who joins up with the “Old Expendables” to battle one of their own. Just like Rousey, Luna is quick with words and handy in a fight.