1. The problem asks: What is a polygon with 3 sides called? 2. The formula or definition: A polygon is a closed figure with straight sides. The number of sides determines its name. 3. Important rule: A polygon with 3 sides is called a triangle. 4. Explanation: - Triangle has 3 sides. - Pentagon has 5 sides. - Decagon has 10 sides. 5. Therefore, the correct answer is A. Triangle.