Question: If someone says wallaahi and is lying but over something very small does he need to fast 3 days? What if he has done it so many times that he can't remember?
Answer: Swearing by Allah on a lie (something one knows is not true) is an enormous sin. One must repent immediately by feeling sorry and intending never to return to something similar. There is no kaffaarah (atonement) in swearing by a lie according to Abu Hanifa.
... and Allah knows best.