(3) Given the sequence $\left\{a_{n}\right\}$ satisfies: $a_{1}$ is a positive integer,
$$
a_{n+1}=\left\{\begin{array}{ll}
\frac{a_{n}}{2}, & a_{n} \text { is even, } \\
3 a_{n}+1, & a_{n} \text { is odd, }
\end{array}\right.
$$

If $a_{1}+a_{2}+a_{3}=29$, then $a_{1}=$ $\qquad$