The three leaders met March 8 and 9 at the Sochi ski resort Krasnaya Polyana.
In his letter, Mitrokhin notes that Berlusconi is under investigation for a number of criminal offenses connected to corruption and says there may be a "corrupt relationship" between the trio.
"The leadership of the country is demonstrating an exclusive relationship with a representative of the foreign private sector, which may indicate the presence of an illegal relationship," Mitrokhin wrote.
The Yabloko leader also noted that "contrary to established practice, the websites of the president and prime minister did not specify the purpose of the meeting, but reported only that it occurred."
The nature of Berlusconi's relationship with Russian leaders has been questioned before. In a diplomatic cable published by Wikileaks in 2010, U.S. Ambassador to Italy Ronald Spogli said Berlusconi was overly sympathetic to the Russian leaders because of a personal interest in joint gas projects.
When the cable was made public, Berlusconi denied that personal interests affected his relationship with Russian leadership.