Mark Dixon, 43, Nicholas Harris, 45, and Matthew Higgins, 33, were found guilty of conspiracy to defraud by a jury at Bristol Crown Court.
They were jailed for seven, eight and six years respectively.
Dixon, of Ash Lane, Down Hatherley, Harris, of Ash Path, Upton St Leonards, and Higgins, of Nelson Street, all Gloucester, will now face a confiscation hearing of their assets.
All three had previously denied conspiracy to defraud between April 2005 and October 2007.
The court heard they ran two linked firms called Construction Management Development Ltd and CMD Services Ltd.
They conned more than 50 people in Gloucestershire, Worcestershire, the West Midlands, Bristol, Wiltshire, Devon, Cornwall, Oxfordshire, Berkshire, Hampshire, Hertfordshire, Buckinghamshire, Surrey, and Kent.
Co-accused Leighton Docksey, 34, and Lee Ireson, 30, were found not guilty at the trial on 17 January.