Solution (source code)

= Solution

Before $A$'s second throw, $A$ can win either immediately, with <probability> $1/3$, or by passing to $B$ and having $B$ throw $3$, with probability $(1/2)(1/6)$. These are disjoint events, so
$$
\boxed{P=\frac13+\frac1{12}=\frac5{12}.}
$$