The court order is the first such verdict after the military-backed interim regime last month adopted a controversial protest law which requires to seek prior police permit for holding protests.
Ahmed Maher, Ahmed Douma and Mohamed Adel were charged with protesting without prior permission from the authorities, judicial sources said. They were also found guilty of rioting and allegedly assaulting security forces during illegal protests.
The three were fined 50,000 Egyptian pounds (USD 7,100) each, sources added.
The three defendants had supported the ouster of Morsi, whom they accused of betraying the 2011 revolution.
Maher is the founder of the April 6 youth movement that led the revolution against Egypt's three-decade long President Mubarak in January 2011.