Is there any place to keep luggage for 3 days at Beijing Capital Airport while i can travel freely?

Yes, there are.

Terminal 1
The lelf-luggage counters is on the east of Corridor between T1 to T2

Terminal 2
The left-luggage counters are near gate 9 in the arrival hall on the first floor and gate 10 and gate 8 on the departure hall on the second floor.

Terminal 3
Service: Round-the-clock left baggage consignment and packing service
East of public space of Arrival Hall at 2nd floor of T3-C
West of public space of Arrival Hall at 2nd floor of T3-C