The new opposition speaker of the National Assembly, Henry Ramos Allup, swore in the three yesterday, defying an injunction by the Supreme Court to suspend them on the government's request.
The three extra deputies bring the total number of opposition seats in the legislature to a two-thirds "supermajority."
Their swearing-in flies in the face of an injunction by the Supreme Court which has yet to rule on whether the three opposition deputies, as well as one other pro-government lawmaker, can legally take their seats in the assembly, pending accusations of electoral fraud against them.
Most of the opposition lawmakers were sworn in during a rowdy session on Tuesday as Maduro vowed to resist them with an "iron hand."
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Allup said his opposition would within six months propose a way "to change the government by constitutional means."
"Here and now, things will change," he said.
Maduro responded in comments broadcast on television: "I will be there to defend democracy with an iron hand. They will not make me give ground or waver."
But he admitted he planned soon to announce a reshuffle of ministers in his government.
Venezuela has the world's biggest known oil reserves but has suffered from a fall in the price of the crude on which its government relies.
It is in deep recession, with citizens suffering shortages of basic goods and soaring inflation.