The UK's telecom regulator has said Three is the only major UK mobile network to have "refused" to automatically cut its customers' monthly charge at the end of their contract's lock-in period.
As a result, Ofcom said, the firm's subscribers would "overpay" unless they took action to change to another deal.
The watchdog said it had challenged the industry to treat users more fairly.
But Three has said that customers were often happy to stay on the same deal.
"Many consumers are happy with the service that they are paying for and may choose to take no action, particularly if the consumer feels that there is not an equivalent, better value, service that meets their needs," it said in a consultation filing.
"Moving consumers to a default tariff, on an opt-out basis, may drive consumer dissatisfaction and complaints."
By contrast, Ofcom said that the other major mobile companies had given it the following commitments regarding out-of-contract customers:
- O2 and Virgin Mobile - will cut the monthly charge to the equivalent of a 30-day Sim-only deal
- Vodafone and EE - will automatically reduce prices three months after the subscribers' lock-in period expires, but have yet to say by what amount
- Tesco Mobile - will reduce the monthly charge to the best available airtime tariff
The discounts are set be introduced by February 2020.
The regulator said that this would help address a situation in which it currently estimated that 1.4 million out-of-contract mobile phone users were spending an average of just under £11 more per month than if they switched to a comparable Sim-only deal.
This situation had arisen, it explained, because the initial deal had covered both the cost of a handset and its usage.
"We're introducing a range of measures to increase fairness for mobile customers, while ensuring we don't leave existing customers worse off," said Ofcom's consumer group director, Lindsey Fussell, in a statement.
"All the major mobile companies - except Three - will also be reducing bills for millions of customers who are past their initial contract period."