You are right, out of the 3 words, dreadful is probably the most dramatic choice. However, in my opinion, all 3 can be interchangeable in your example sentence... To me "Don't look at me... I look terrible/horrible/dreadful!" Would mean the same, no matter which word you chose :)

"Dreadful" means the same thing as "horrible" and "terrible," but it has a slightly different feel. To me, "dreadful" sounds a little more polished than the other two. Because it is less commonly used, it might sound a little more emphatic, too. For example, if you said, "I've just woken up and I look dreadful!" someone might think, "Wow, she must look really bad!" ^^ So I would use "dreadful" when you want to mean something is extra horrible/terrible, but you want to say it in a nice-sounding way.

thank you guys! it's clear to me now, thx!!!! :3