The three activists chanted "down with the military regime," as the court gave its verdict, official news agency MENA reported.
Maher is the founder of the April 6 youth movement that led the revolt against Mubarak. All three defendants were leading dissidents under Mubarak, but they also supported the military's overthrow of Morsi, whom they accused of betraying the 2011 "revolution".
Maher and Douma were arrested after Maher's supporters allegedly scuffled with police outside a Cairo court on November 30, when Maher handed himself in for questioning on suspicion he had organised an illegal protest.
Adel was absent from the first hearing on December 8 but was captured this week in a midnight police raid on a non-governmental organisation in Cairo.
They were found guilty of violating a disputed law enacted last month that requires police authorisation for protests, less than three years after Mubarak was toppled by massive demonstrations.
Another prominent pro-democracy activist, Alaa Abdel Fattah, has also been arrested for allegedly taking part in a violent and illegal protest. The date for his trial has yet to be determined.
More recently, the military justified its overthrow of Morsi -- Egypt's first freely elected president -- as a response to massive protests against his turbulent year-long reign, which critics said was marked by power-grabbing and economic mismanagement.
Road map 'meaningless'
Amr Ali, general coordinator of the April 6 youth movement, said the court verdict "aims to terrorise political activists so they would stop demonstrating against the failings of this regime".
He said the "road map" outlined by the authorities for Egypt's transition to democracy "has become meaningless".
"The road map was to put in place a government which was to be the foundation of a democratic state, but what we see is a return to the practices of the repressive state of Mubarak," Ali told AFP.
"We don't recognise the roadmap. We appeal for a demonstration tomorrow (Monday) and on January 25 against the protest law and to demand the release of pro-democracy activists."
The road map envisages a referendum on a new constitution next month, followed by parliamentary and presidential elections by the mid-2014.
The new protest law has angered many secular and liberal activists who saw the military-installed government as a lesser evil than Morsi.
Human Rights Watch's Egypt director said Sunday's verdict was an "indication" of what could come in the future.
"The verdict is significant because it sits in very much with the pattern that we have seen, particularly in the past three weeks," Heba Morayef told AFP.
"This is a standard Mubarak era tactic to crack down on protests ... they (the authorities) are particularly targeting protest leaders."
Since Morsi's overthrow, authorities have launched a sweeping crackdown on his supporters that has left more than 1,000 people dead and thousands more in jail, including virtually the entire top leadership of his Muslim Brotherhood, which prevailed in a series of polls held after Mubarak's ouster.
On August 14, security forces stormed two pro-Morsi sit-ins in Cairo, sparking clashes in which hundreds of people were killed, mainly Morsi supporters, in the worst mass killing in Egypt's modern history.
On Sunday, Egypt's interim president Adly Mansour issued a decree to set up an "independent fact-finding committee that will investigate the incidents that happened from June 30 onwards," a statement issued by his office said, referring to the day when millions poured onto the streets demanding Morsi's resignation.
In a separate development, another pro-democracy activist from the city of Suez, Bassem Mohsen, died on Sunday, two days after he was shot during clashes, medical sources and MENA reported. Mohsen had been jailed during Morsi's presidency.