A cylinder seems to fit the description
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It could be a cylinder
A triangle has to have straight sides
A three-sided shape with one curved side is known as a "curvilinear triangle." This shape consists of two straight sides and one curved side, creating a triangle-like form with a rounded edge. It can be used in various mathematical and design contexts, where a blend of linear and curved geometry is required.
This is called a heptagon.
A shape with straight sides is called a polygon. But it can have 3 or 4 sides as well.