Any two sides of a triangle must add to more than the third. If they do not, the sides cannot close. 3, 4, 8 fails because 3 + 4 is not greater than 8. Angle sum is a different triangle fact, in triangle angle sum.
Check the longest side
Add the two shorter sides. If that sum is not bigger than the longest, it is not a triangle. Exterior-angle facts assume a real triangle, in the exterior-angle theorem.
Lines and angles as a set are in lines and angles. The short list is in ACT Math formulas.
The bottom line
Two shorter sides > longest side. Equality is a flat line, not a triangle.
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