The three leaders with all their forces would not represent...
The three leaders with all their forces would not represent a great danger against the unity of the nation if they were alone challenging the authority of the Imam. The Imam would be able to subdue their forces easily if they were alone without Muawiya. Their rebellion was extemporaneous. They did not have what Muawiya had of resourcefulness strong army and wide popularity in a large Islamic province.
As a matter of fact people of Basra were divided in their loyalty towards them and a party from the people of Basra fought them as soon as they entered and before the Imam came to confront them. Muawiya on the contrary was dangerous by himself even if the three leaders were not with him. He was wily and resourceful. His movement was not extemporaneous. He prepared for it for two decades. He was in a province whose inhabitants were obedient to his order.
He had a huge and well organized army ready to comply with his command. He was capable of challenging the authority of the Imam even if he did not have any helpers from outside Syria. Therefore the biggest concern of the Imam after he finished the Battle of Basra was to prepare for facing this enemy who was dangerous to the unity of the nation and its future.
To leave no excuse for Muawiya the Imam sent him a message with Jareer Ibn Abdullah Al-Bajali inviting him to join the majority of the Muslims who elected him. He mentioned in that message that those who elected him were those who elected Abu Bakr ‘Umar and Uthman and that the right of the decision about the caliphate was only for the migrant and the Medinite companions. If they elected a man their election would be binding to the rest of the Muslims.
If anyone tries to break away by accusation or innovation they would bring him back. If such a person persists in his secessional way they have to fight him for taking a road other than the road of the believers. The Imam also said to Muawiya in this message the following: "Talhah and Al-Zubayr elected me then they broke their covenant with me. Their breach of covenant is like their faith desertion.
I fought them after I left no excuse for them until the truth came and the cause of God prevailed while they were averse. "I invite you to join the Muslims by pledging your allegiance to the new administration.