Waheed Ali, 24, Sadeer Saleem, 27, and 31-year-old Mohammed Shakil all deny one charge of conspiring to cause explosions between January 1 2004 and July 8 2005.
They are alleged to have worked with Mohammed Sidique Khan, Shehzad Tanweer, Jermaine Lindsay and Hasib Hussain in the run-up to the quartet's suicide attack on London.
The four bombers, all from West Yorkshire, murdered 52 innocent people when they triggered explosions on the capital's transport network on July 7 2005.
Mr Justice Gross yesterdaytold 150 potential jurors that they would be asked to complete a 16-point questionnaire to help him select a 12-member jury for the trial at Kingston Crown Court.
He said: "As you know, Mohammed Sidique Khan, Shehzad Tanweer, Jermaine Lindsay and Hasib Hussain were responsible for causing a series of explosions on the London Underground and public transport system on July 7, 2005, that killed 52 people and injured hundreds of others.
"It is alleged that Waheed Ali, Sadeer Saleem and Mohammed Shakil assisted these men by, in particular, conducting reconnaissance of possible targets in December 2004."
Mr Justice Gross was expected to select the final 12 from a panel of 30 today.
The prosecution, led by Neil Flewitt QC, is expected to open its case tomorrow.
Ali, Saleem and Shakil deny conspiring to cause by explosive substance an explosion to endanger the lives or likely to cause serious injury to persons or damage property in the UK.
Sidique Khan, a 30-year-old father-of-one who grew up in Beeston before moving to Dewsbury, is regarded as the ringleader of the attack on London. Tanweer, 22, came from Beeston, while Hussain, 18, was from Holbeck. Lindsay, 19, originally came from Huddersfield but later moved to Luton.
paul.robinsons@ypn.co.uk