L&Q, The Hyde Group and East Thames said the proposed move will turn them into one of the country’s top four largest housebuilders with a value of around £30bn.
By joining forces they will also have the firepower to build 35,000 more homes than they could have achieved alone, the trio said, as well as delivering around £50m of cost savings.
Half of the 100,000 new homes will be for people on lower incomes, making the new organisation the largest provider of affordable homes in the country.
Around 25,000 affordable new homes will be set aside for first time buyers while another quarter of the homes will be for affordable rent, with the remaining 50,000 for market rent and sale.
The extra 35,000 homes built as a result of the merger represent £9bn of investment and £25bn in total for the 100,000 homes.
Housing Minister, Brandon Lewis MP said: “This shows what can be achieved by combining the strengths of each organisation. It will make a real difference to increase their capacity to build, house and help thousands of people across London and the South East.”
The new organisation will also draw on East Thames' expertise and invest in a new offer for vulnerable and older people, catering for London’s ageing population. A new training academy with an extra £5m a year to spend on training, skills and providing apprenticeships will also be created.
David Montague, chief executive of L&Q, has been lined up as chief executive of the new group, while Hyde Group boss would become deputy chief executive. Yvonne Arrowsmith will continue as chief executive of East Thames, which will become a subsidiary of the new organisation.
The three housing associations currently manage or own around 137,000 homes with developments currently underway at Barking Riverside and Nine Elms and Heron Fields in Sittingbourne, Kent.