In a statement today, Three said that it added 301,000 customers in the last quarter (July to September), which it claims makes it "the UK's fastest-growing mobile operator".
More than 1.04m customers have joined Three over the 12 months ending September 30, bringing its base to 8.8m. However, the company still lags far behind EE, which is the market leader with more than 27m customers.
Alongside its "smartphone focus", Three said that its growth is down to a nationwide programme to upgrade its network.
Three is deploying superfast DC-HSDPA technology around the UK to increase the speed of its backhaul, the data pipes that connect sites to the internet.
This is part of the company's new Ultrafast network experience, which currently covers 39% of the UK, but will reach over half the population by the end of 2012, and more than 80% by April 2013.
Customers in existing coverage areas are able to use DC-HSDPA-enabled devices, such as the iPhone 5, iPad mini, Sony Xperia T and Windows Phone 8X by HTC, supporting reported mobile data speeds over 20Mbps.
Three also recently purchased a portion of EE's 1800Mhz spectrum, which more than doubled the firm's available capacity.
Dave Dyson, the chief executive of Three UK, said: "We have led the adoption of smartphones in the UK by giving our customers great coverage, fast speeds and compelling price plans.
"These network improvements are designed to maintain our position as the fastest-growing UK mobile operator. Our Ultrafast network is giving more people in more places access to an even faster mobile internet experience.
"But mobile internet is about more than just technology. All-you-can-eat data at prices to suit everyone and peace-of-mind bundles that guarantee no-surprise data bills are designed to allow customers to get the most enjoyment from their smartphones."
Three launched a major recruitment drive in October that will add nearly 400 sales and service roles at its Glasgow office, and 400 new employees across its high street stores.