Exeter Crown Court heard on Friday that the men were working on a site building new student accommodation for Exeter University at the former Elmfield Nursery in New North Road when the incident happened in February 2008.
Cowlin Construction Ltd was fined £85,000 and ordered to pay £20,000 in costs by the court in the case brought by the Health & Safety Executive (HSE).
Prestoplan Ltd, which provides timber frame buildings, was fined £35,000 and ordered to pay £22,000 towards costs.
Somerset Carpenters Ltd, which supplied labour at the site, was fined £50,000 with £20,000 costs.
Somerset Carpenters was served with a prohibition notice by the HSE following an inspection at the site which banned them from working until safety measures had been put in place to stop workers falling down the lift shaft.
A wooden platform was then erected over the shaft but two weeks later this collapsed, sending Ricki Slocombe (35) and Matthew Blackmore (29), both from Bridgwater, falling to the ground floor.
Mr Blackmore suffered a broken back and Mr Slocombe suffered two broken legs and had to use a wheelchair for several months. He has been unable to return to work since then.
Speaking after the hearing, HSE inspector, Simon Chilcott, said: "This was a terrifying incident for the two men, which could easily have let to their deaths.
"As it was, they suffered severe injuries, which were life-changing in the case of Mr Slocombe.
"Contractors and employers must make sure they have taken all reasonable measures to provide a safe environment for workers to work in and that any temporary structures are secure."
Cowlin Construction Ltd of Cater Road, Bishopsworth, Bristol; Prestoplan Ltd, of Turnpike Road, High Wycombe, Buckinghamshire and Somerset Carpenters Ltd, of Bath Road Business Park, Bath Road, Bridgwater, Somerset each pleaded guilty to a breach of Section 3(1) of the Health and Safety at Work Act 1974.