The measurements would be 180 for 3 sides, 540 for 5 sides and 1080 for 8 sides.
You can find the sum of any regular figure using the interior angles formula. The formula is below for you.
(n - 2) * 180
In this equation, n is the number of sides. So the work is done below for you for each number of sides.
3 sides
(n - 2) * 180
(3 - 2) * 180
(1) * 180
180
5 Sides
(5 - 2) * 180
(3) * 180
540
8 Sides
(n - 2) * 180
(8 - 2) * 180
(6) * 180
1080
