The BB in 3BB refers to Big Blind. Any number that comes before BB is how many times the big blind the raise is. So in this example, to raise 3BB in poker means to raise three times the big blind.
It's short for "good game," something poker players traditionally say when they bust from an online tournament, or to someone after they have busted, complimenting their play. It's also said sarcastically, pointing out someone's flawed play or unfortunate bust-out.