Why is it bad?!
It is prescriptively incorrect from the standpoint of Standard English, but descriptively correct in certain nonstandard dialects.
I'd have helped if I'd known. Third conditional.
I could have helped if I had have known
It is not a standard construction in English.
I would have helped, had I known.
2. I'd helped if I had known.
3. Had I had the knowledge, I would have helped.
'Had have' is not English.