"Open 3" is one of my pet peeves. Even as a seasoned orienteer I have no idea what it means. It usually refers to the course number (as people have guessed above), but for beginners and people from outside Canada who don't know the course structure, this isn't much help. Not to mention that we have different course structures for different size events.
For the upcoming Alberta Champs (Sept 16/17 in Canmore - don't miss it!) I've recently been using the new Orienteering Canada course guidelines
and I'm really impressed. They've mostly made changes so that juniors have several options to progress (from a fast track, to more casual). They've also given actual names to the open courses (such as beginner, intermediate, expert, and so on). Plus they give a Technical Difficulty chart. This should help newcomers a little.