That's a good question, and I can't point to specific studies that put a precise number on it. I will have to look up Laura Stephenson's study.
One thing I would say is we can talk about short-term effects, where you encounter a particular electoral context and ask who you should vote for. There's also the question of longer-term effects. It seems as if certain parties, the ones we know, are the ones that realistically have a chance of winning. Therefore, allegiances might start, over time, to erode for the smaller parties that seem to never get much representation and never have a chance of making it to the big ring or gaining power.
I don't know Laura's study. When I hear 3%, that does sound low to me. I would think more people are affected by this issue, this problem. As I say, I would probably draw that distinction between short-term and longer-term influences as well.