Augustine Gbao, who the court said was the RUF's ideology trainer, will spend 25 years in prison.
The men were convicted in February of ordering and carrying out a spree of killings, rapes and mutilations during Sierra Leone's decade-long war.
The three are the first RUF fighters to be tried by the court, which has already jailed members of a pro-government force and those of a separate armed group formed by members of Sierra Leone's former military rulers.
By the time the Sierra Leone's civil war ended in 2001, thousands of people had been killed and tens of thousands more had had their arms, legs, noses or ears cut off.
According to a summary of the judgement in February, the three men were part of a so-called joint criminal enterprise aimed at gaining "political power and control over the territory of Sierra Leone and in particular the diamond mining areas".
During the trial, the prosecution argued that the RUF needed the "blood diamonds" to fund their war against the government.
Hundreds of thousands of people were forced to work in the RUF-held diamond mines during the conflict.
The RUF established control by "terrorising the civilian population by massively killing innocent civilians, pillaging and burning houses of those they considered and branded supporters of the corrupt government", the prosecution said.
The judgement also invokes widespread rape, sexual violence and "short sleeved and long sleeved amputations" - where victims were asked to choose between short sleeves, meaning amputation of the arm at the shoulder, or long sleeves, amputation of the hand at the wrist - by RUF fighters.
The sentencing of the three RUF leaders marks the final stage in the last trial to be held in Freetown by the Sierra Leone tribunal.
Both the prosecution and the defence can still launch an appeal which will also be heard in Freetown before the court closes its doors in the Sierra Leonean capital permanently.
Charles Taylor, the former president of neighbouring Liberia, is on trial on charges of fomenting Sierra Leone's conflict during his own country's 1989-2003 civil war. His trial is underway in The Hague.