Finnish Hakaristi roundels, for aircraft from 1918 through to 1944.
These decals are scaled to represent 30", 36" and 48" roundels in 1/300 scale. (They're also ideal for large aircraft in 1/600; equivalent to 60", 72" and 96" diameter.) The actual sizes are 2.5mm, 3mm and 4mm.
Obligatory disclaimer; please note that the Hakaristi is not a swastika, and has nothing to do with Nazis; it was on Finnish planes at least a decade before them.