I think everyone in Year 11 should do a 3/4. Personally, I saw a lot of mates this year do one in Year 11 and get pretty average scores (high 20's, low 30's) when they expected 35+. It really is a wake up call, especially if you do not attend an "elite" school where they push you to do well. Students realise that it is difficult to get high scores unless they put the effort in. I know I'd rather wake up to this in Year 11, instead of doing so on results day in Year 12.
Effort, there are different ways people can put in effort, some people need to put in lots of effort to do well while others pass easily. My point is, you should be doing FOCUSED study, not pointless time wasting. Everyone learns differently, find out the way you STUDY and LEARN best, ensure you are actually taking in the information you are learning and commit it memory.
A study score of 31 still means you beat 50% of the state. Consider it a tool to prepare you for Year 12 though, now you hopefully understand the work required to do well in VCE.