Ionel Soci, 34, was crushed to death when a wooden frame used for holding poured concrete collapsed on him in Higher Fraddon, Cornwall.
His 28-year-old wife Monica was pregnant with their second child.
FLI Energy, PJL Construction (Scotland), and Premier Construction, all admitted offences under the Health and Safety Act at Exeter Crown Court.
Judge Robert Linford adjourned sentencing until a date to be fixed next month.
Any fines of FLI or PJL will never be paid because they are both in administration and any fine will become an unsecured liability, the court heard.
The court heard Mr Soci was working on the Greener for Life plant at Penare Farm on 11 June 2014.
He had been helping to pour concrete and was dismantling the wooden shuttering when it fell on him.
Michael Forster, prosecuting, said there was "no effective supervision at all" and Mr Soci had not received a proper safety briefing.
Angus Withington, for PJL, said there were experienced staff on site and nobody could have anticipated Mr Soci doing what a fellow worker described as "an act of madness".
Mr Soci was described by his family in Romania as "a hero who gave his life on the altar of work".
Builders of a biogas plant have admitted safety breaches which led to the death of a Romanian labourer.