A 3 on 2 is very similar to a 2 on 1 in many ways. You need to communicate with your defenseman, challenge the shooter, and don't over commit. But there are a couple of things that are different about the two.
On a 3 on 2 you always want to make sure that you are challenging the shooter because the other two should be tied up by your defenseman. Try and stay standing for as long as possible just in case the shooter tries to go around you. He probably won't be able to because of your 2 defenseman, but it's better to be safe than sorry. Have confidence in your defenseman. You got to know that he is taking the guy without the puck. If there happens to be a pass across make sure that you get across the crease as fast as possible for the big save.