The Flying Phantom sank in freezing fog in December 2007 while towing a cargo vessel on the River Clyde near Glasgow.
At the High Court in Glasgow today, tug company Svitzer Marine Limited pleaded guilty to a series of health and safety breaches including failing to put in place a safe operating procedure following the grounding of the Flying Phantom in foggy conditions in a previous incident in December 2000.
Stephen Humphreys, 33, from Greenock, Inverclyde, Eric Blackley, 57, from Gourock, and Bob Cameron, 65, from Houston, Renfrewshire, died when the tug sank.
A fourth man, Brian Aitchison, 37, from Coldingham in the Borders, was rescued.
The accident happened when The Flying Phantom was towing the Red Jasmine vessel in the Clyde Harbour area on the evening of December 19.
Dense fog severely restricted visibility and the crew could not see the boat they were towing.
The larger vessel overtook the tug and caused it to capsize.
Svitzer Marine admitted "failing to ensure, as far as reasonably practicable, the health, safety and welfare at work" of the crew.
The Flying Phantom suffered £150,000 of damage when a vessel it was towing on the Clyde in 2000 collided with the tug in similar circumstances, the court heard.