Solution (source code)

= Solution

For $B$ to make a first throw, $A$ must first pass, with <probability> $1/2$. For $B$ then to win on that throw, $B$ must roll one of two winning faces, with probability $1/3$. The throws are <independent>, giving
$$
\boxed{P=\frac12\frac13=\frac16.}
$$
A first throw of $3$ by $A$ would make $B$ win without throwing, and is excluded by the requested event.