3.(1)设A,B,C是三个事件,且 P(A)=P(B)=P(C)=1/4, P(AB)=P(BC)=0, P(AC)=1/8, 求A,B,C至少有一个发生的概率。(2)已知 P(A)=1/2, P(B)=1/3, P(C)=1/5, P(AB)=1/10, P(AC)=1/15, P(BC)=1/20, P(ABC)=1/30, 求 A cup B, overline(AB), A cup B cup C, overline(ABC), overline(ABC), overline(AB) cup C 的概率。(3)已知 P(A)=1/2, (i) 若A,B互不相容, 求 P(overline(AB)), (ii) 若 P(AB)=1/8, 求 P(overline(AB)).

(1) 由概率加法公式，$P(A \cup B \cup C) = \frac{5}{8}$。
(2) 依次计算得：$P(A \cup B) = \frac{11}{15}$，$P(\overline{A}\overline{B}) = \frac{4}{15}$，$P(A \cup B \cup C) = \frac{17}{20}$，$P(\overline{A}\overline{B}\overline{C}) = \frac{3}{20}$，$P(\overline{A}\overline{B}C) = \frac{7}{60}$，$P(\overline{A}\overline{B} \cup C) = \frac{7}{20}$。
(3) (i) $P(\overline{A}B) = \frac{1}{2}$，(ii) $P(\overline{A}B) = \frac{3}{8}$。

答案：
(1) $\boxed{\frac{5}{8}}$
(2) $\boxed{\frac{11}{15}, \frac{4}{15}, \frac{17}{20}, \frac{3}{20}, \frac{7}{60}, \frac{7}{20}}$
(3) (i) $\boxed{\frac{1}{2}}$，(ii) $\boxed{\frac{3}{8}}$