The original source of "there are three unfilial sons, and no posterity is great" is Mencius and Li Lou. The context is "there are three kinds of unfilial, no queen, it is a big deal, you marry without telling, because there is no queen, the gentleman thinks it is still telling."
Shun, the leader of the Chinese nation, did not report to his parents in advance when he married his wife. This practice is considered as not fulfilling the obligations of the younger generation.
But in fact, Shun married his daughter only after Yao inspected Shun's virtue. So Shun didn't tell his parents in advance, but in the eyes of a virtuous gentleman, Shun's practice is the same as telling his parents in advance.
As can be seen from the above, China has always had a misunderstanding about the saying that "there are three unfilial things, and there is no great thing after". The real meaning of this sentence is to encourage and persuade the younger generation to do their due duty.