the last do can be understood He runs as fast as I run He runs as fast as I do He runs as fast as I these 3 sentences are all correct and mean the same thing

Yes, it needs the last do. Specially if you're writing it. Sure, you can say that sentence without "do" in a casual conversation but it's not as correct as saying "as fast as I do". Why is that? In English you should (in most cases) have a verb right after the pronom. If you leave the pronom without a verb it's almost like the sentence is not finished. Example: Someone asks you: - Do you like going to the beach? And your reply is: - Yes, I. It's not correct, you see? Sure, the person you talk to will understand, but they will notice right away that English is not your first language.

It is preferred to use "do" at the end of this sentence. However, if you just said the original sentence "He runs as fast as I" it can still be understood.