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."
Brian: If only I'd listened to my folks and hadn't dropped out of school, I could've been someone by now.
Cecil: Woulda coulda shoulda, Brian.