Arrive in beijing aiport T3 from US air china & connecting flight 90 minutes later to Hong Kong.

So you are traveling with Air China for the whole journey? If so, 90min is enough for transit and all transfer procedures can be finished within T3. All guiding signs are clear for you and no visa is needed. As I know, it is not needed to go through the immigration and they will only stamp on your boarding pass at the customs counter. Take it easy!