The jury is still considering verdicts against three other men facing the same charges.
All six defendants denied conspiracy to murder and conspiracy to cause explosions likely to endanger life.
The convicted trio claimed that the bombings were a hoax, designed as a protest against Britain's involvement in Iraq but not to kill.
Eritrean-born Ibrahim, the self-confessed bomb-maker, said the bombs were deliberately designed not to explode but only to go "pop".
Omar and Mohammed tried to set off their devices aboard two underground trains, and a couple of hours later, Ibrahim's device failed to explode on a double-decker bus.
The explosives were packed in plastic tubs, with screws, bolts and other pieces of metal taped to the outside as shrapnel. The detonators contained triacetone triperoxide (TATP), an explosive used by the July 7 bombers.
Police said scientific tests on the devices proved that they were all viable. They did not know why they did not work.
Much of the prosecution's case was based on eyewitness testimony as well as closed circuit television footage from the targeted underground trains and the bus.
In one piece of footage, Mohammed attempts to detonate his charge with his backpack facing a mother and young child.
Moments, later passengers are seen running to the other side of a carriage, while an off-duty firefighter remonstrates with Mohammed.
Police believe that the planning for the attack started after Ibrahim returned to Britain from a trip to Pakistan in March 2005. He was in the country at the same time as two of the July 7 bombers - Shezhad Tanweer and Mohammed Sidique Khan - but officials do not know if they ever met.
They believe the transit system was not the original target, but was chosen following the successful attacks two weeks earlier. Their original target is unknown.