You can choose when you want to take the 3 hours. So if you start right now, you would stop after 3 hours (and then go about publishing on itch). But, you can also start tomorrow at, say, 7:00 AM, and keep going till 10:00 AM. Actually, you can even break the time into parts -- as long as the total time you spend working on the game (art, programming, and music) fits within 3 hours.