Answer
Circumference (circum=around): "length $around$ the circle".
Diameter (dia=passing through). "distance $passing~through$ the circle".
Radius (ray=spoke): "The length of a $spoke$ from the center of a circle to the edge".
Work Step by Step
We look at the prefixes in each geometry term:
Circumference. Prefix: Circum. Meaning: around (e.g. circumvent). Thus circumference means "length $around$ the circle".
Diameter. Prefix: Dia. Meaning: passing through (e.g. diagonal). Thus diameter means "distance $passing~through$ the circle".
Radius. Prefix: Rad. Meaning: ray or spoke (e.g. radiant). Thus radius means "The length of a $spoke$ from the center of a circle to the edge".
