Call the length of the third side $n$. By the triangle inequality, $1+3>n$ and $1+n>3$, or $2<n<4$. The only integer $n$ which satisfies this is $\boxed{3}$.
Question:
A triangle has sides measuring 1 unit and 3 units. The length of the third side is an integer value of units. What is the length of the third side, in units?
Answer:
Let's think step by step.

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A triangle has sides measuring 1 unit and 3 units. The length of the third side is an integer value of units. What is the length of the third side, in units?
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