noun which may take the genitive case particle 'no'
Describes something shaped like a crescent moon. Often used with の to modify nouns.
三日月形のパンを焼いた。
I baked crescent-shaped bread.
湖は三日月形をしている。
The lake is crescent-shaped.
三日月 is the noun for 'crescent moon' itself, while 三日月形 is the shape or form of a crescent.
Compound of 三日月 (crescent moon) + 形 (shape).