Since quadrilateral A'B'C'D' is a translation of quadrilateral ABCD, the corresponding sides of the two quadrilaterals will have the same lengths. This means that:
- \( AB = A'B' \)
- \( BC = B'C' \)
- \( CD = C'D' \)
- \( AD = A'D' \)
Given that \( BC = 3 \) units, we can conclude that the length of \( B'C' \) is also \( 3 \) units.
Therefore, the answer is:
C. 3 units