Third Reich https://www.flaggenlexikon.de/fdtlhi3r_me.htm
The German Empire is sometimes called ‚Third Reich‘ for the period of the dictatorship of the NSDAP. Its flag showed the colors black, white and red until 1935; and until 1945 a red flag with a white disc and a black swastika in it.
and merchant flag, ratio = 3:5, Source, by: Uniform-Fibel