What is the third side of an isosceles triangle if 2 sides measure 3 and 13 inches

-- It's an isosceles triangle, so two of the sides are equal.

-- So the missing side must be either  3  or  13.

-- If the missing side is 3, then you have two 3-inch sides
joined together at one end. 
If they're completely open and pointing in opposite directions,
then they add up to 6 inches long. 
==> They're not long enough to reach both ends of the 13-inch side.
So the missing side can't be 3.

-- It must be 13 inches.

An isosceles triangle has two equal sides that are usually longer, so the answer is 13¨