The suspects were testing the strength of remote-controlled devices, which were intended for use at mass protests, the police said on Saturday, but it was unclear which chemicals or explosives were involved as the bombs had been detonated, the South China Morning Post reported..
Acting on intelligence, officers from the organised crime and triad bureau ambushed the trio in Tuen Mun on Saturday morning as they carried out tests.
Officers also seized a radio-controlled detonation device and protective gear, including shields, bulletproof vests, a steel plate and gas masks at the scene.
The tools were believed to have been used during the tests.
"The amount (of explosives) was not a lot. But intelligence showed there were two purposes behind the plot - one was to upgrade the power of the bombs, and the other to launch attacks at future assemblies or rallies," Senior Superintendent Steve Li Kwai-wah said.
Saturday's operation came as police said three men and two women, aged 15 to 18, had been arrested in suspected connection with the death of a 70-year-old man who was hit by a brick during a fight between masked protesters and Sheung Shui residents last month.
Earlier in the week, police defused two powerful home-made bombs in Wan Chai.
Separately, police said on Saturday that they found 34 petrol bombs, 20 smoke bombs, 12 corrosive bombs and a number of easily flammable items after receiving a call from City University staff about suspected dangerous items on the Kowloon campus, reports the South China Morning Post newspaper.
The university also discovered dangerous chemicals on campus on December 6, which were disposed of by police's explosive ordnance disposal bureau.
( With inputs from IANS )