The Isle of Man network, built in partnership with Siemens and NEC, is currently made up of 24 transmitters covering 85 percent of the 76,000 people on the 30-mile (48km) by 10-mile (16km) island.
First trials will be limited a small number of users, said Manx Telecom, with dates for a full roll-out depending on availability of handsets. This could be as late as 2003.
Mobile groups have sunk billions of euros on 3G licences, putting a spotlight on the small self-governing island better known for offshore finance and the TT motorcycle race.
The companies hope 3G will lead to a big boost in data revenues as customers download video clips or play games on networks much faster than dial-up connections and about two-thirds as fast as broadband DSL.
Users with laptops would be able to use their 3G phone as a high-speed, portable modem, allowing them to play sophisticated games and download large files. MmO2 says the testing period will determine which applications are best suited to 3G.
"There will probably be 20 different 'killer apps'," Manx Telecom managing director Chris Hall told reporters.
MmO2's 3G networks will cost Ã,£4billion to build, though it has little debt and expects 3G network-sharing deals with Deutsche Telekom and KPN Telecom will save Ã,£1.2bn in capital expenditure over 10 years.
The Isle of Man trial comes as BT Cellnet promotes the RIM Blackberry wireless email device and mmO2 prepares to launch the O2xda mobile phone and data device early next year. The trials has faced several delays; it was first due to begin in May, but was delayed because of technical problems.
"We said at the time of the postponement that we were delaying the launch until late summer, and that's still the plan. No specific date has been set yet," the company spokesman insisted at the time.
Reuters contributed to this story.
