Meaning: would have, could have, should have
A reply you give to a whinging
person who is expressing
regret for not having taken a particular course of action when the opportunity originally presented itself. They've suffered the consequences and they are possibly
fishing for sympathy or simply beating themselves up about it.
It essentially means, "Too late for regrets, you can't change the past, get on with your life, learn from the experience, make better choices in future, stop doing a post mortem on your life."