HELP - Dulles International Arrival to Domestic Departure

International Arrival Frankfurt > Dulles D Gate (Lufthansa - Star Alliance) 3 Hour Layover before Departing domestic on United (Star Alliance) from A Gate

3 American Citizens and One Chinese Citizen in our party.

Can someone walk me through how we go from deplane Lufthansa to boarding the UA domestic flight.

Baggage??? Does Lufthansa tag the bag through to final domestic destination or do we need to claim bags at international baggage claim?

Customs???? Separate customs lines for Americans and Chinese??? Can we all clear customs together?

How do we get from International Gate D to Domestic Gate A TSA screen and what must we do in between and in what order and where???

What "mistakes" should we be wary of that would delay the process and jeopardize making our domestic flight in the 3 hour window we have???

Thank you!!!