The Health and Safety Executive launched the prosecution after Tony Laughton, David Jowett and Shaughan Walsh were killed during the refurbishment of a mill building in April 2000. The three were underneath a wall when it collapsed.
It later emerged that the HSE had issued a prohibition notice two months before the collapse.
Hugh Davies, the HSE's head of operations, who led the investigation, said that construction safety was a priority for the executive. He said that the dangers associated with refurbishing old buildings, particularly when demolition was involved, were well known.
Davies said: "This case should serve as a warning to others who may be tempted to cut corners and who fail to comply with a prohibition notice."
The company faced prosecution for seven offences under the Health and Safety at Work Act. Thadani faced prosecution on two counts.
This case should serve as a warning to those tempted to cut corners
Hugh Davies, HSE head of operations
It has endorsed 18 practical safety guides for designers, which can be seen at www.safetyindesign.org.
Working with the Construction Industry Council, the Institution of Civil Engineers and CIRIA, the HSE plans to target the architectural sector to improve safety.