What is the best way to support those who have gotten notice of a 3rd deployment?

You could call your congressman, but the military really doesn't like when you do that. It screws things up then they get super pissed at you and screw you over in other ways. When you're in the military, you should expect these things. It does suck, but all you can do is be there for him. Try to talk with him about it. I would just let him know that you love him and he's going to be ok.