What is the 'three-second rule' in defensive driving, and how should it be adjusted when driving a bus in wet weather?

1. It is the time taken to check all safety mirrors, and it remains constant
2. It is the minimum safe following distance in seconds, and it should be doubled to at least six seconds in wet weather
3. It is the time allowed to clear an active bus lane, which decreases during rain
4. It is the duration of the amber traffic light, which increases in wet weather