It is also certain to deepen the rift between Egypt's liberal and secular pro-democracy campaigners — many of whom participated in the 2011 popular uprising against Hosni Mubarak — and the current military-backed government.
The ongoing campaign against dissent and pro-democracy activists has been overshadowed by a much larger crackdown against the Muslim Brotherhood group and other Islamists. That push has led to the death of hundreds and the jailing of at least 16,000 people since the July ouster of President Mohammed Morsi.
The ruling sparked a storm of anger on social media networks, with dozens of activists calling for a sit-in outside the presidential palace in the Cairo suburb of Heliopolis. The wife of Ahmed Douma, one of the three activists, wrote on her Twitter account that she was headed there. Douma suffers from stomach ulcer and has complained in past court hearings that he was not being properly treated in jail.
The three activists — Ahmed Maher, Mohammed Adel and Douma — were leading figures in Egypt's 2011 popular uprising that toppled long-time autocrat Mubarak.
The three were accused of violating a controversial new law on holding street protests and were sentenced each to three years in prison in December.
The law imposes draconian restrictions on any public gathering, but the government says it is meant to bring order and stability to the streets in the face of months of continued protests by supporters of the Islamist Morsi.
The court also ruled on Monday that each of the three would be fined 50,000 Egyptian pounds (about $7,140).
"The judge, regretfully, has upheld the sentence on Douma, Maher and Adel. Down, down with the oppressive verdict," one of their lawyers, Gamal Eid, wrote on his Twitter account, minutes after the ruling was read by presiding Judge Ahmed Faisal.
Another defense lawyer, Ahmed Seif al-Islam, said he planned to challenge the ruling, but acknowledged that the process could take months.
"If we can't overturn the verdict, we will take the case to the African Court on Human and People's rights," he said.
The three young activists were the founders of the April 6 movement, which was at the forefront of the 18-day uprising against Mubarak, but later joined the opposition against the generals who took over the reins of power for nearly 17 months after Mubarak's ouster.
April 6 initially supported Morsi, Egypt's first freely elected president, but soon became disillusioned with the Islamist leader's perceived attempt at concentrating powers with his Brotherhood.
The movement supported Morsi's removal by the military last July, after days of protests by millions of Egyptians against his rule.
But again, April 6 shifted its stance to denounce what it describes as the curtailment of freedoms and the heavy handedness of the police under the current military-backed government.
The predominantly pro-military media has cast the April 6 movement as a treasonous organization with foreign links.
Monday's development comes as Egypt is heading toward a crucial presidential election next month in which the front-runner is former military chief Abdel-Fattah el-Sissi, who led the ouster of Morsi.
Associated Press writer Maggie Michael contributed to this report from Cairo.