These are all grammatically correct. I personally like the last one best as a standalone sentence. The first two sort of imply that the sentence will go on to explain why you are not a student anymore: I used to be a college student, but then I was abducted by aliens. The second sentence sounds a little more natural if it were I have/had been a college student before.

All are correct, but i preferably like the first one 😊

"I used to be a college student."

The only one that I think is doubtful is the second one. I was a student before..... before what? It would be simpler to say "I was a student."

Depending on the context they could all be correct.