Maher, Mohamed Adel and Ahmed Douma were charged with organising an unauthorised and violent protest in November, days after the passage of the law.
The sentencing in December of the activists, members of a broad coalition of groups that supported Morsi's ouster, had raised concerns of a return to Mubarak-era repression under the new military-installed regime.
The ex-army chief who overthrew Morsi, Abdel Fattah al-Sisi, is running for president next month. He has promised there will be "no return" to the practices of Mubarak's regime, which was toppled by an Arab Spring uprising after ruling Egypt for three decades.
The retired field marshal is expected to easily win the May 26-27 vote as he is riding a wave of popularity for ending Morsi's divisive year in power.
Secular-leaning activists have increasingly been targeted in an extensive crackdown on the opposition, mainly Islamists, that has seen an estimated 15,000 people jailed.
Another prominent leftwing activist, Alaa Abdel Fattah, is standing trial on similar charges to those the trio faced.
Ahmed Seif, a lawyer for Maher and his co-defendants, said they will appeal the ruling and, if that fails, will take the case to the African Court of Human and People's Rights.
"There are no indications that the state is willing to loosen its grip," Seif told AFP after the ruling.
Human Rights Watch criticised Monday's verdict.
"Today's verdict against three of the most recognised faces of the January 25, 2011 protests is one more nail in the coffin for Egypt's revolution," Sarah Leah Whitson, HRW's director for the Middle East and North Africa, said in a statement.
And in Washington State Department spokeswoman Jen Psaki said the United States was "deeply troubled" by the court decision.
"Their continued imprisonment under a law that severely restricts the universal right to peaceful assembly and expression runs counter the Egyptian Government's commitment to fostering an open electoral environment and a transition process that protects the universal rights of all Egyptians," she said.
"We urge the Egyptian Government to exercise its constitutional authority to commute these excessive sentences, which are not in line with the rights guaranteed in Egypt's new constitution, Egypt's international obligations, or the government's own commitment not to return to Mubarak-era practices."
Critics say the government has given police a free hand to clamp down on dissent, amid a crackdown that has seen more than 1,400 people killed in street clashes since Morsi's overthrow.
Courts have tried hundreds of Islamists en masse, with one court sentencing 529 to death for allegedly participating in a deadly riot. That sentencing is likely to be overturned on appeal.
Militants have meanwhile unleashed a deadly campaign against security forces that has killed nearly 500 policemen and soldiers in bombings and shootings since Morsi's ouster.
The government said last week it had boosted punishment for "terrorist" offences and expanded the scope of the crimes that fall under that category.
The legal amendments will come in force when interim president Adly Mansour approves the government-penned draft.