The probability all three fail is $p_1p_2p_3$.
The probability at least one does not fail is $1 – p_1p_2p_3$.
Explanation
- (a) By independence, the probability that all three components fail is the product of their individual failure probabilities: $P(\text{all fail})=p_1\cdot p_2\cdot p_3$.
- (b) “At least one does not fail” is the complement of “all three fail”, so
$P(\text{at least one does not fail})=1-P(\text{all fail})=1-p_1p_2p_3$.
Notes / variants
- If each component has the same failure probability $p$, then $P(\text{all fail})=p^3$ and $P(\text{at least one does not fail})=1-p^3$.
- You can also express part (b) in terms of success probabilities $s_i=1-p_i$ by expanding the complement if needed, but the compact complement form above is simplest.