"LeT has received a major blow because three LeT hardcore terrorists ... have been sentenced under POTA [Prevention of Terrorism Act] for bombing Mumbai, killing people and terrorising people."
Nikam said the crimes were "of a very serious nature and ... we are going to ask for the harshest punishment".
The judge said the three would be sentenced in early August.
The three defendants had pleaded not guilty to all charges and Sushan Kunjuramaran, their lawyer, said he was shocked at the court's decision and would consider an appeal.
Two other men accused of providing and assembling the high-explosive RDX devices used in the attacks were previously acquitted by the court.
The bombs were planted in two taxis and exploded within minutes of each other in two busy areas of Mumbai at lunchtime on August 25, 2003.
At least 2,000 Muslims had been hacked, beaten, shot or burned to death during the violence in Gujarat the previous year, which erupted after 59 Hindu pilgrims died in a train fire which was at first blamed on a Muslim mob.
A subsequent inquiry concluded the fire was accidental.