It happened outside the Kirti Monastery in Aba county, where many of the self-immolations have taken place.
This brings the number of Tibetans who have set themselves on fire since 2009 to 51, the group added.
The BBC is unable to verify this figure. Foreign media are banned from the region, making reports hard to confirm. Chinese state media have confirmed some of the self-immolations but not all.
The young men were shouting slogans against Chinese rule and policies in Tibet as their bodies burned, Radio Free Asia (RFA) reported, citing witnesses.
"Witnesses saw them run about 20 steps with their bodies on fire, and then they fell to the ground," two monks based in India told RFA.