Iftikhar Ahmed Chaudhry, Islamabad inspector-general of police, said on Tuesday that the suspects were seized on Monday, based on information provided by eight alleged militants detained earlier in the month after the three foiled attacks.
"They are educated. One of them is son of a retired army brigadier, one of them is an engineer who was technically aware of making circuits," he said.
Police said the men were picked up in the industrial sector of the capital when police officers intercepted a car, adding that they were still being interrogated.
"They are hardcore terrorists," Chaudhry said.
Sikandar Hayat, a senior superintendent of police, said the trio - named only as Ali Ahmed, Muneer and Khalil - were the driving force behind the plot and the eight others arrested earlier were only facilitators.
He did not specify who was the brigadier's son, but said Ali had likely done a masters degree in engineering.
"This is the core group. We can call them masterminds," he said.
Officials had previously said the plot had loose links to the al-Qaeda network.
One rocket exploded on October 4 in the Ayub public park in Rawalpindi, a garrison city adjoining Islamabad, near the army residence of General Pervez Musharraf, the president. Another three were found nearby.
Two more rockets were found near the official presidency building and parliament in Islamabad on October 5, followed by another two near the intelligence service headquarters in the capital two days later.
Officials say the plot had loose
links to the al-Qaeda network
Aftab Sherpao, the interior minister, had said the men planned to launch all of them simultaneously, but only the one in Rawalpindi worked.
Security forces traced the suspects by decoding mobile phones attached to shells.
Police seized more of the Russian-made rockets plus grenades, explosives and hundreds of sniper rifle rounds at the same time as they detained the initial eight suspects.
Musharraf, who seized power in a coup in 1999, has said the rockets were possibly aimed at him. He has escaped at least three serious assassination attempts in the past, one of them involving members of the Pakistan Air Force.