Indeed. If the lowest score you give for anything is a 3, you may as well score on a 3-7 scale and I'll just manually subtract 2 from all your scores.
Not that it has to average out, that for every 5 there is a 1, that would be silly - but it IS a curve, 1 is the worst, 5 is the best. Everything else, falls in between. I think 5 is a bit restrictive, as there is a lot of grey area between "average" and "WOW" for 4 to cover, but it's all we have for it, so of course some will go in 5 who aren't quiiiite as strong, and some will go in 3 that are just a touch above the average.
Please also remember not to focus TOO much on whether or not they can defend against a rush, or how hard it is to play them to their strengths - MP readiness and SP intuitiveness are taken care of by the other 2 ratings, and people need to keep that in mind.