The opposition urged its supporters to gather at the home of Falah al-Sawwagh, one of the three MPs, to protest the verdict and to stage a demonstration at night.
"The verdict against Khaled al-Tahus, Falah al-Sawwagh and Bader al-Dahum is three years each with immediate effect," lawyer Mohammad al-Jumia wrote on his Twitter account.
The ruling means that the three former MPs must go to prison immediately without even waiting for their appeal to be heard.
The former lawmakers were charged of undermining the status of the emir during an address at a public gathering on October 10 in which they had warned that any amendment to the electoral law could lead to street protests.
"This is a political ruling," former MP and opposition leader Mussallam al-Barrak, who is facing similar charges, said after Tuesday's verdict was issued.
"They have breached the constitution and played with the election system ... now they want to terrorise us ... we will not surrender and will not be scared," Barrak told opposition supporters at the house of Tahus.
In a separate statement the opposition urged the "Kuwaiti people to gather at the house of Sawwagh" later Tuesday.
Former Islamist opposition MP Khaled al-Sultan warned that "politicising the judiciary" could trigger violent reactions and held the government responsible for the consequences.
The information ministry said Kuwait has a "transparent and independent judicial system".
"All citizens, regardless of their position, are equal in the eyes of the law. Anyone accused of a crime in Kuwait will get a fair trial with a comprehensive legal defence and open appeals process," said the ministry.
Kuwait has seen many opposition-led demonstrations in protest against the amendment to the electoral law which they claimed allowed the government to influence election results and elect a rubber-stamp assembly.
The Kuwaiti opposition even boycotted the December 1 general polls in protest against the amended law.
Mohammad al-Humaidi, director of Kuwait Society for Human Rights, confirmed Tuesday's verdict and said what the defendants spoke at the gathering was "more of an advice rather than a criticism."
"There is no clause in the Kuwaiti constitution that bars people from addressing the emir directly and advising him," Humaidi told AFP.
Under Kuwait's constitution, it is illegal to criticise the emir.
There was no immediate reaction from the former MPs who were detained for six days in October and then released on a bail of $17,850 each after the first hearing.
During that hearing they strongly denied the charges levied against them, saying they had spoken within limits of the law.
Tuesday's verdict is not final and can be challenged in the court of appeals and the supreme court.
OPEC member Kuwait which produces around 3.0 million barrels of oil per day, has been rocked by ongoing political disputes since mid-2006 that have stalled development despite abundant surpluses.