The main thing to keep in mind that Trijam was designed for improving your skills and do regular, quick iterations. There are many benefits that come with that, but you can use all the work you have done before, you can use 3rd party assets, etc. 3 hour challenge is for executing the project you have planned. You can plan as much as you'd like before the jam. That's another thing - you develop planning skills and then see if you can scope the project properly. And also, you can go over the 3 hours. It's mostly just a challenge, but to have at least some motivation to beat the challenge, only games that are made within 3 hours can go into hall of fame if they win. Otherwise, you can spend all the time you'd like, not a problem to submit a game. Actually, it would be better to submit a game that you have made in 10 hours than not to submit at all.