A cylinder.
Also, howsabout a hemisphere ?
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A cone
its a lens...1 curved edge = a circle, 1 flat surface = back, 1 curved surface = front. (these are used in some battery-less flashlights, as an example.)
A cylinder. It has circular ends with a rectangle curved between them.
No 3D shape has only three sides to be a complete unit. If a 3D shape has a side which is curved, it has more than three sides, because any curve on a 3D object is an infinite number of sides. Think of it like if you had a million sided 2D object. From a distance, it would look like a circle, but mathematically, you can't have a perfect circle.
A 3D semi circle