Each paid out £5,000 to have their logos included on a timeline in the Museum of London's new galleries, which will feature works from more than 300 years of the capital's history.
The works will span from 1666, the year of the Great Fire, to 2012, the year of the Olympic games.
BLP bought 2001, the year Berwin Leighton and Paisner & Co merged creating a firm of more than 900 staff including 122 partners. Since then the firm has almost doubled its turnover and now employs about 1,200 people.
Freshfields bought 1743 as the firm can trace its lineage back to that year when Samuel Dodd was appointed solicitor to the Bank of England. The magic circle firm has acted for the bank ever since.
Guy Morton, senior partner at Freshfields, said: "We chose 1743, the year of Freshfields' genesis, and I'm looking forward to seeing our name taking its place in the timeline of the City's history."
Slaughters purchased 1889, the year two young solicitors, William Capel Slaughter and William May, left their erstwhile law firm to set up on their own.
The Museum of London kick-started the initiative to raise funds for its £18m redevelopment. The timeline will be on display in 2009 as part of a project that will transform the lower floor of the Museum of London to tell the story of the city over the past 300 years through the lives of Londoners.
The displays will contain objects such as Nelson's jewel-encrusted sword, Suffragette banners and an Ozwald Boateng suit.