HRW's statement said that "the letter indicated that they were tortured with electric shocks and beaten in the prison".
Deputy Director of the International Organization for the Middle East and North Africa, Michael Page, called on Egypt to "ban the execution of death sentences, which amplifies the cruelty of unfair trials".
Egyptian authorities have banned public funerals for the three young men and imposed strict measures on the private funeral, which was attended by few relatives who prayed for them in the mosque at dawn and buried them quickly.
Egypt ranked 126th in world for freedom
US organization Freedom House has ranked Egypt 126th out of 195 countries when it comes to public freedoms.
According to a new report by the organization based on 2018 data, Egypt and Sri Lanka "are among the least free countries at the level of Internet Freedom".
The report explained that "Egyptian President Abdel Fattah el-Sisi continues to rule Egypt in an authoritarian manner," pointing out that serious opposition to the president "no longer exists, as liberal and Islamic activists have been constantly prosecuted and imprisoned".