Having Harry Potter's name come out of the Goblet of Fire was quite a surprise, not least to him. If Ron weren't so thick, he'd have known for sure that Harry had nothing to do with it. Had it been me, I'd have spun some story about how I'd figured a way around the Age Line, and put all three of our names in. I can think of several ways, such as using an owl to drop them or bribing an older student. Harry, unfortunately for him, is no actor and not good at thinking of things like that on his feet. I'm rather surprised that all those years with the Dursleys didn't force him to develop those skills in self-defence, but he says they'd have done him no good; the Dursleys blamed and punished him for anything "unusual" no matter whether he was innocent or not.