Assume that the prg takes inputs as 3 sides of triangle : 

1. Enter 0 0 0 : Error : no triangle can be made
2. Enter -1 -1 -1 Error : no triangle can be made -ve values
3. Enter 1 0 x : Error : no triangle can be made . only 
numbers and no alhpabets
4. Do not enter any values : Prompt to enter valid values 
5. Enter 0.000000000000001 , 1000000000000,1000000000000 : 
Triangle should be made (Boundary value)

6. And the most tricky 
Enter 1 2 4 : Triangle can not be made . Summation of 2 
sides should always greater than third value . 1+2<4 . So 
triangle can not be made 
7. 2 2 2 : Equilateral triangle 
8. 1 2 2 : Isoscelous triangle 
9. Test with very small values , very large values and 
combinations of both .

First check whether it is triangle or not by "any two sides 
sum should be greater than third side"
check if 3 sides are equal
check if a NE to b NE to C
 check if any two sides are equal

Some more possibilities r there,
like sides u can also write TC for the angles