Revolutionary United Front leaders Issa Sesay, 38, and Morris Kallon, 45, were convicted of 16 of the 18 charges against them, and Augustine Gbao, 60, on 14 counts, the BBC reported.
They are to be sentenced at a later date, the British network said.
The charges against the rebel trio arose from allegations of rape, mutilation and killing of civilians during Sierra Leone's 10-year civil war, which began in mid-2004. About 120,000 people were killed while tens of thousands were left mutilated.
The judges concluded the men "significantly contributed" to a joint criminal enterprise with former Liberian President Charles Taylor to control the diamond fields of Sierra Leone to finance their warfare.
They were also found guilty of enslaving young girls and forcing them to marry rebels.
Taylor's case before the United Nations-backed Special Court for Sierra Leone has been moved to The Hague in the Netherlands for security reasons. He faces 11 counts.