Question

What is the sum of all measures of the interior angles of a regular three-sided polygon?
a. 100°
b. 150°
c. 180°
d. 360°

Answer 1

The sum of the interior angles of a polygon is:

180(n-2)

Where n is the number of sides. Therefore

180(1)
=180

So the interior angles add up to 180 degrees in a triangle.

Answer 2

The sum of the three angles inside a 3-sided polygon...
commonly referred to as a "triangle" ... is 180 degrees. 

It doesn't matter whether the polygon is regular or not. 
Any three sides make three angles that total 180 degrees.