IN competitive internal medicine fellowships, I think it does matter. I had people in my class -- similar Steps I, II -- similar med schools -- not get some competitive cardiology/GI interviews as their step III was not as good -- it was about a 10% difference really, but the big factor we could come up with was that Step 3 scores were different -- one kid took it amidst busy rotation, barely passed, other had scores on average to previous.
I think the competitive fellowships have a "filter" that kicks you out if your score is like "really" low......so that might be how the score does matter. But, for most people, I think not a huge thing about it.