International Arrival--International Departure at PEK with No Checked Luggage

Yes, you need to go through the custom. But you needn't to do immigration and quarantine.

The correct procedures are Flight Arrival - Transfer Procedures - Border Inspection - Security Check - Custom Inspection - Flight Waiting and Boarding.
Generally speaking, it takes at least two hours transfer time. However, if your arrival and departure both are in T3, the minimum transfer time is about an hour.