Why 3 minutes
Three minutes is a soft-boiled egg with a genuinely runny yolk, a round of amateur boxing, and the cooking time on most instant noodles. It is the shortest interval where a timer really earns its place — long enough to start another task and forget, short enough that forgetting ruins the result.
It will not drift while you do something else
The countdown is worked out from a fixed end time rather than counted down frame by frame, so putting the tab in the background, locking your phone, or switching apps does not cost you any of the 3 minutes. Browsers throttle background JavaScript to save battery, and that is exactly what makes most browser timers lose time. Press start, go and do the thing you are timing.