Stephen Rapp, the chief prosecutor, told Al Jazeera that the verdicts "respect the suffering of the multitude of victims who were murdered and enslaved or mutilated and raped or rendered homeless.
"It does so by holding individual leaders were responsible for some of most serious crimes known to humankind," Rapp said.
Earlier Rapp said in a statement: "[The court] recognizes the very deep and long lasting suffering inflicted upon women through conscription as 'bush wives' during the Sierra Leone conflict," he said.
"It sends a message that may deter such attacks against the men and women who are protecting individuals, restoring security, and keeping the peace across the globe," he said.
Sesay, 38, and Kallon, 45, were found guilty of 16 charges and Gbao, 60, was convicted of 14 counts against him.
They are expected to be sentenced in March.
More than 50,000 people died in the war in Sierra Leone