The Party of Life, the Pensioners' Party and the populist Rodina (motherland) Party said they sought to challenge the dominance of the Kremlin-loyal United Russia Party.
The leader of the Party of Life, Sergei Mironov, is an ally of Vladimir Putin but said: "We will be in opposition, in particular to United Russia ... This is an historic event. This political union will change fundamentally the political map of Russia."
The Pensioners' Party and Rodina have also been linked to the Kremlin, but Mironov dismissed as "invention" claims that the coalition was stage-managed by the governmet.
Vladislav Surkov, the Kremlin deputy chief of staff, floated the idea of the coalition in March.
He created United Russia and is seen as the architect of a system that some commentators have described as "managed democracy".
Surkov said: "Society does not have a 'second leg' that it can switch to if the first leg has gone numb. That makes the system unstable."
Russians vote for a new parliament next year in what is likely to be a dress rehearsal for the presidential election.
The new coalition said it would represent "the working man" and press ministers to spend more of the billions of dollars it has accumulated from oil revenues.
Those revenues are being banked in a rainy-day fund to avoid overheating the economy, a policy praised by Western economists but disliked by many voters who cannot understand why their incomes are not rising more quickly.
"Is this coalition genuine? No," said Maria Lipman of the Carnegie Moscow Centre, a think tank.
"The Kremlin is leaving nothing to chance so nothing unexpected can undermine the transition of power, however it is orchestrated, in 2008."
The name of the new party is expected to be "Homeland, Pensioners and Life - Union of Trust".
It looks likely to diminish further the power of the Russian Communist Party, which garnered only 12.7 per cent of votes in the elections to the State Duma in 2003.
Gennady Zyuganov, the Communist Party leader, said the new party was "completely devoid of prospects".
However, Sergei Markov, a Kremlin-linked analyst, said it could realistically upset the status quo in parliament.
United Russia is the dominant force in Russian party politics.
The Communists are the second-biggest party but analysts say they pose no real threat. Other parties are either very small or toe the Kremlin line.