By default, the challenge coins are 2D coins if without further notice.
If consider enamel pin as is 1.0 then challenge coin should be 2.0. Challenge coin is sort of the upgrade version of enamel pin: Most of pins are one side pin, while most of coins are two sides coin. We classify challenge coin as : one side (color fills) coin, two sides (color fills) coin, no color coin, one side 3D coin, two sides 3D coin.
One side 3D coin means there is 3D effect on one side of the coin. The other side is 2D. Colors can be fills on both/either sides or it also can be no color.