Does "By God, I will not forgive you" necessitate expiation after forgiveness, and is fasting three days sufficient as expiation?

An oath uttered unintentionally is considered a 'laghw' (idle/vain) oath, and there is no sin or expiation for it, based on the Almighty's saying: "Allah will not impose blame upon you for what is unintentional in your oaths, but He will impose blame upon you for what you intended in your oaths." There is a difference of opinion regarding the definition of 'laghw'. Some say it is swearing about something believing it to be true, only for it to turn out otherwise. Others say it is what flows from the speaker's tongue without intention.