Feel free to ask!
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.
That's right, just as Bojidar explained! :)
You find time during weekend of about 3 hours, make a game (theme: no visuals; so you can be creative with it), have fun! Planning what you want to make is not in the 3 hours, exporting builds is not in the 3 hours, it's just the development! However, if you want to take more than 3 hours, it's ok. We play and give feedback to all games. The ones that fit in the challenge are eligible to be crowned as the winners of Trijam that week :)