So you're asking a cultural question, rather than a grammatical one.
Here is my opinion.
Don't use the first two. I feel it means essentially this:
I am so amazing that you're lucky to have me as a friend. I'm not a very great friend, but the privilege of being my friend worth my failings. I feel sorry for you, actually, because you want to be my friend so badly you'll put up with my bad behavior.
The third one isn't quite right, because you're not really sorry. If you were, you would change your ways.
So your original thread, with the "I'm sorry, but" is right -- it's a polite filler. You acknowledge your failing.