You mean to say you went 28 races without getting nailed by a blue shell at the end? I envy you.
I 3 starred all cups in the game first try as well. It has nothing to do with not getting hit. I got hit by a blue shell once almost every race and twice on two races. You just need enough of a lead so that it doesn't matter if you get hit of fall off the map a few times... and that's not very hard when you're versing Mario Kart AI made for an audience of mostly little kids lmfao.
Constant Blue and Red Shells while in first, then getting hit with another item on recovery multiple times was always the reason my big lead quickly turned to 5th place in nearly every fourth race. But overall, it was a considerable challenge, especially since it took me a while to learn tracks.