Step 1: Apply Basic Proportionality Theorem (Thales' Theorem).
According to the Basic Proportionality Theorem, if a line parallel to one side of a triangle intersects the other two sides, then the two sides are divided proportionally.
In the given figure, $PQ \parallel BC$, so according to the theorem:
\[
\frac{AP}{PB} = \frac{AQ}{QC}
\]
Step 2: Use the given ratio.
We are given that $PQ : BC = 1 : 3$. Hence, the ratio of $AQ$ to $QC$ is also $1 : 3$.
Thus, we have:
\[
\frac{AP}{PB} = \frac{1}{3}
\]
Step 3: Conclusion.
Therefore, the ratio of $AP : PB$ is $2 : 3$.