British prosecutors said they couldn't immediately confirm whether or not they will seek a retrial in the case.
The three men, who acknowledged at their trial that they knew the four London suicide bombers, are the only people so far charged over the attacks on the capital's transit network that killed 52 commuters on three subway trains and a bus.
Following 15 days of deliberations, a jury at a London court said it could not agree on verdicts and was discharged.
Waheed Ali, Sadeer Saleem and Mohammed Shakil all denied a charge of conspiring with the bombers to cause explosions.
In evidence at the trial, prosecutors alleged the men had taken part in a test run in December 2004 for the attacks, claiming they had joined three of the eventual bombers to scout out targets including subway stations and a host of tourist sites.
Neil Flewitt, prosecuting, claimed the men had cased out possible targets in London, including the Natural History Museum, the iconic London Eye ferris wheel and the London Aquarium.
He said that Ali, 25, Saleem, 28, and Shakil, 32, visited a series of locations which bore a striking similarity to where the bombs were detonated on July 7, 2005.