كُلَّ ثَلَاثَةٍ عَلَى بَعِيرٍ means that three people shared a camel by turns. This was a practical consequence of limited transport. A fair arrangement protected everyone’s opportunity to ride.
Abu Lubabah and ʿAli wished to give up their own comfort as an act of service. The Prophet’s ﷺ answer shows that leadership is not a claim to all available conveniences. Distributing work according to circumstances, and not burdening others unnecessarily, is also an ethical responsibility.
Fairness and willingness to serve are complementary in this account: the companions offered help, while their leader remained willing to share the hardship.