LA Fitness, Dave Whelan Sports and Harlands Group will extend rights to allow members to cancel their contracts early if a change in circumstances makes attendance difficult or unaffordable and be more upfront and transparent about initial membership periods and cancellation rights.
They will also stop describing contracts as being of a fixed duration if they automatically continue on a rolling basis after the initial period expires.
LA Fitness and Dave Whelan Sports have almost 500,000 members between them and Harlands Group manages more than 900,000 customer contracts.
The moves follow an agreement earlier this year by Bannatyne Fitness, David Lloyd Leisure and Fitness First Clubs to change their contract terms.
The OFT is also writing to 20 other health and fitness operators to highlight contract terms and commercial practices which could be unfair, and advising them to review these.
OFT executive director Cavendish Elithorn said: "As a result of our investigation, millions of gym members now have better contract terms, including improved cancellation rights that prevent them being unfairly locked in if their circumstances change. T his should be a warning to the rest of the industry that we will not hesitate to take action where we find evidence of consumers being treated unfairly.
The OFT said it would continue to monitor compliance alongside Trading Standards.
The OFT launched its investigation after a High Court ruling against a gym management company found lengthy membership periods could be unfair when they locked people in without adequate cancellation rights if their circumstances changed.