Aloha :)
$$p_{a1}=\frac{\binom{8}{8}\binom{24}{0}}{\binom{32}{8}}=\frac{1}{10\,518\,300}$$
$$p_{a2}=\frac{\binom{8}{4}\binom{8}{4}\binom{16}{0}}{\binom{32}{8}}=\frac{70\cdot70}{10\,518\,300}=\frac{49}{105\,183}$$
$$p_{a3}=\frac{\binom{16}{8}\binom{16}{0}}{\binom{32}{8}}=\frac{12\,870}{10\,518\,300}=\frac{11}{8990}$$
$$p_{b}=\frac{\binom{4}{1}^8}{\binom{32}{8}}=\frac{65\,536}{10\,518\,300}=\frac{16\,384}{2\,629\,575}$$