Three hours is a morning, a lab practical, or a long cook. You will not stare at it, so the alarm matters. Leave a note on the timer page about what 'done' is. Future you, three hours older, is worse at remembering.
Wake lock keeps the screen on, which matters if the device would otherwise sleep and hide the digits. Press space to start or pause, R to reset, and F for fullscreen. The screen can stay awake after you start, using the wake lock your browser allows. Pick the alarm before you begin. A bell, a chime, a beep, or a short digital pulse all come from the Web Audio API, so there is no sound file to wait on.
This page is the 3 hours countdown and nothing else is running underneath it. Related lengths are linked under the guide if you want a neighbor, and the timer itself stays above the fold.
Questions
How do I run a 3 hours timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. Wake lock keeps the screen on, which matters if the device would otherwise sleep and hide the digits.
What is 3 hours actually good for?
Three hours is a morning, a lab practical, or a long cook. You will not stare at it, so the alarm matters.
Will the countdown drift if I switch tabs?
No. The end is a timestamp. The display is the gap between that timestamp and now, so a background tab does not lose seconds the way a ticking counter would.