Police also arrested prominent Shiite cleric Sheikh Hussein al-Maatuq on the same accusation and he is expected to be brought before prosecutors on Monday, defence lawyer Abdulkarim bin Haider told AFP.
Several hundred Shiites hold a protest outside the secret service headquarters, south of Kuwait City, where Maatuq is detained. The crowds dispersed peacefully after hundreds of special forces were called in.
The two former MPs in custody are Abdulmohsen Jamal, who turned himself in on Sunday, and Nasser Sorkhouh who was arrested last Tuesday. Also in custody is municipal councillor Fadhel Safar.
The crackdown on Shiite activists follows a rally last month to mourn Hezbollah military commander Imad Mughnieh, who was killed in a car bombing in Damascus.
However Haidar said prosecutors did not question the suspects about the rally but instead focused their questioning on allegations that they joined a previously unknown group the authorities say is called Hezbollah Kuwait.
"They accused them of joining Hezbollah Kuwait to spread ideas for the destruction of the basic foundations of the society through illegal means, which amounts to overthrowing the government," the lawyer said.
"They were also accused of spreading false reports that weaken Kuwait’s foreign position," Haider added.
So far, seven activists have been questioned. All have denied the accusations.
Formal charges will only be pressed when prosecutors have completed their inquiries.
Haider said most of the suspects were questioned and cleared of similar accusations about two decades ago at the height of the 1980-88 war between then Sunni-dominated Iraq and Shiite Iran.
The rally and subsequent arrests have fuelled sectarian tensions in this oil-rich Gulf state where about a third of the citizen population of one million are Shiites.
