"Three family slaves" is a derogatory term for Lu Bu's disloyalty and injustice, meaning that he betrayed three fathers (including his biological father and two stepfathers)**The following are the details:
1.*Biological father and Ding Yuan**: Lu Bu's biological father died early, and he later recognized Ding Yuan as his father-in-law, but in the sixth year of Zhongping (189), by Dong Zhuo's **, Lü Bu betrayed and killed his father-in-law Ding Yuan.
2.*Dong Zhuo**: Later, Lu Bu surrendered to Dong Zhuo and was accepted by Dong Zhuo as his adopted son. However, in the later historical development, because of the Diao Chan incident, Lü Bu betrayed again and killed his second father-in-law, Dong Zhuo.
3.*Moral Condemnation**: In ancient China, where loyalty and filial piety were emphasized, betrayal of one's father was regarded as a great unfilial piety and disloyalty, so Lü Bu's actions were severely morally condemned. In "Romance of the Three Kingdoms", Zhang Fei scolded Lu Bu as a "slave with three surnames" in the battle, which was the biggest humiliation of Lu Bu's personality and an accusation of his disloyalty and injustice.
To sum up, the title of "three family slaves" is not simply because Lu Bu changed his lord several times, but because he betrayed his biological father and two fathers-in-law, which was considered extremely immoral in the social context of the time.