1: For the first one, this is how I would phrase it:
A girl as beautiful as you are would never date me
This is because the subject is "A girl", and you are describing it as "as beautiful as you are".
Another storm as violent as the one yesterday would destroy the house!
This all goes with the basic structure:
You can have "More...than", "Less...than", and "As...as"
Subject + Be Conj. + (first) + Adjective + (second) + Object
For instance: The boy is MORE energetic THAN the cat!
OR: The computer is AS complex AS a human brain!
Note using superlative you can miss the first comparative word
The man is faster than you!
2: Again, I would use the same structure as said above:
I met a girl as beautiful as you are at the library yesterday.
The same reasons stated in sentence 1, you phrase the subject "a girl", then "as beautiful as you are" to describe the girl you met. Then you can talk about extra details "at the library yesterday".
I met a person as tall as the tree in my garden yesterday!
3: You have written this correctly!
I'm not as beautiful a girl as you are.
This is because, you are saying that you are not as beautiful as the girl in question, and this goes before the subject.
Another example could be:
I'm not as strong a man as you.