There are four species within the genus Eurema in Japan: Eurema blanda, Eurema brigitta, Eurema hecabe and Eurema laeta.
The Common Grass Yellow (Eurema hecabe) looks very similar to the Three-spot Grass yellow (Eurema blanda).
The difference, however, is on the underside of the forewing. In Eurema blanda there are three small spots and a fourth bigger one in the middle; in Eurema hecabe there are two small spots and one bigger spot in the middle.
Yoyogi Park, Tokyo, Japan, 22 September 2018