These small series of 3 stamps  was one of the first series of stamps issued by Germans for the use in General Government. They were issued on 28th of October 1940 to commemorate the first anniversary of establishing the General Government (administrative unit ruling occupied Poland). 
 
You can receive the following configurations of these stamps:
a strip of 5 identical stamps (red, green, or purple)
a strip of 3 stamps (one of each) 
 
These beautiful stamps were designed by a German painter and artist Otto Engelhardt-Kyffhäuser. He was an SA, SS and NSDAP member, which certainly helped him get some business. 
Before August 1940 original Polish stamps would have been overstamped with Nazi eagle, denomination and "Generalgouvernement".