The gang had planned to set off up to eight rucksack bombs and possibly other devices in crowded places. Khalid even boasted that their plan could be "another 9/11".
He, Naseer and Ali were all told that they faced life sentences for the plot.
Six other men, who are also from Birmingham, had already admitted terror offences. They are Rahin Ahmed, 26, from Moseley; Mujahid Hussain, 21, from Yardley; Naweed Ali, 25, Ishaaq Hussain, 21, Khobaib Hussain, 21, and Shahid Khan, 21, all from Sparkhill.