All three sentences mean the same thing. You are correct that the third sounds informal, but that is not a correctness issue. And your second sentence does sound a little strange because most people would leave out at if they're already leaving out I am.
The reason for your concern is that shortening some sentences can allow ambiguity (which is not the case in your examples). Consider the following sentence:
I like Jane more than Mary.
Does that sentence mean I like Jane more than I like Mary? Or does it mean I like Jane more than Mary likes Jane?
It is unclear without the additional words.
Now, consider the following:
I like peas more than carrots.
In this sentence, it is clear that I prefer peas to carrots. It is impossible for the sentence to mean I like peas more than carrots like peas, because it is well-known that carrots are incapable of liking peas.
Your example is more like the peas and carrots sentence than the Jane and Mary one. After all, Spanish is not capable of being better at English than you are.