The network said 5G usage on its network more than quadrupled last year and has already surpassed 3G network use.
It said shutting down 3G would free up resources to improve 4G connectivity and continue its 5G rollout, with more than £2 billion committed to the UK.
Three said the demand for more data continues to increase and rose to 19GB per month per customer in December – a rise of 20% on the previous year – meaning there is a need to invest in newer, faster and more efficient technology.
5G is considerably faster than previous generations of mobile connectivity and is able to deliver larger amounts of data faster.
The company said it expected 5G to account for 35% of data usage by the end of this year.
“3G kick-started the mobile revolution – and launched Three into the UK 20 years ago – but the future is undoubtedly 5G,” David Hennessy, Three UK’s chief technology officer, said.
“As we continue to roll out our ultrafast connectivity, by not only upgrading our existing 4G sites but building new 5G sites, we’ll be in a position to switch off our use of 3G across our network by the end of 2024.
“As the leader in 5G deployment and the fastest network as rated by Ookla, we are focused on delivering our goal of providing better connectivity, every day, for every customer.”
Three said it would work with customers with older 3G-only and other incompatible devices as the switch-off date approached to ensure that they stay connected.
Earlier this year, Vodafone also announced that it would begin switching off its own 3G network in 2023.