I'm not sure what you mean.  Here's a 5-pointed star, but I don't
know how to make a 5-pointed star out of three triangles.

Did you mean this kind of 5-pointed star, also known as a pentagram?  



If so, we'll extend the lines like this:



In the center is a 5-sided regular polygon (a regular
pentagon).  The sum of the interior angles of a polygon
is gotten by the formula:

SUM OF INTERIOR ANGLES = (NUMBER OF SIDES - 2) * 180°

For a 5-sided polygon (pentagon) this is (5-2)*180° = 3*180°=540°

Since all 5 angles of a regular pentagon are equal, each
interior angle of the regular pentagon is ° = 108°

So I'll mark one of the 108° interior angles of the pentagon:



Its suppplement is found by subtracting 180°-108°=72°.
We'll mark it 72°:



That 72° angle is one of the base angles of an isosceles
triangle.  So we'll mark the other base angle 72° also.



Now we can find the angle at the top point of the star by
adding the two equal base angles and subtracting from 180°.

72° + 72° = 144°
180° - 144° = 36°

So each point of the star is 36°.




You wanted the sum of the points interior angles of 
the points.  There are 5 of them, so 5 times 36° is

180°.

Edwin