Step 1: Understanding the given figure.
In trapezium $ABCD$, the sides $AB$, $PQ$, and $DC$ are parallel. The lines $AD$ and $BC$ are transversals. Hence, the ratios of the corresponding segments on these transversals are equal.
\[
\frac{AP}{PD} = \frac{BQ}{QC}
\]
Step 2: Substitute the given values.
\[
\frac{3}{12} = \frac{BQ}{14}
\]
Step 3: Simplify the equation.
\[
\frac{1}{4} = \frac{BQ}{14}
\]
\[
BQ = \frac{14}{4} = 3.5
\]
Step 4: Conclusion.
Hence, $BQ = 3.5$ cm.
Correct Answer: $BQ = 3.5$ cm