Keep in mind, the jam lasts the whole weekend, so you can plan out what you want to build and find a good time to execute - planning and submitting is not in the 3 hour challenge! :)
It's about getting practice under time pressure! If you go over the 3 hour limit, it's fine, still submit your game! The real benefit is if you do it regularly - you'll get better at focusing, planning, execution and you'll have a bunch of prototypes you can then choose to develop further or take that experience to build something bigger! :)
