The sum of the length od any two sides of a triangle is always greater than the third side
Let T be the thirs side of the triangle
3 + 12 > T ----------------------------(1)
3 + T > 12 -----------------------------(2)
12 + T > 3 --------------------------------(3)
The first equation states that T < 15
The second equation states that T > 9
Since the third equation will give us a negative number and there is no negative length, we will ignore the third equation
Putting the first and second equation together,
9 < T < 15
This implies that the third side could be any number that lies between 9 and 15