The third side is 13 inches
Since this is an isosceles triangle, 2 of it's sides are equal.
Hence sides could be 3, 3 and 13
or 3, 13 and 13
Given the 3 sides of a triangle
For the sides to form a triangle, then the sum of any 2 must be greater than the third
Check 3, 3 and 13
3 + 3 = 6 < 13 ⇒ not a triangle
Check 3, 13 and 13
3 + 13 = 16 > 13 ← this is repeated for 3 and the other 13
13 + 13 = 26 > 3
Thus 3, 13 and 13 form a triangle with third side 13 inches