Giulio Andreotti today accepted a new mandate to form another government three days after a withdrawal of Communist support had forced his last government to fall.

His Christian Democratic party offered the Communists, and the three smaller parties that had supported Andreotti since last March, an unprecedented role in putting together a new government. But the Christian Democrats maintained their firm opposition to bringing the Communists into the government itself.

"It certainly will not be easy," Andreotti tod reporters after meeting President Sandro Pertini, who had asked him to try to pull together a new government.

For the first time, the Christian Democrats, who have provided Italy's prime ministers since World War II, agreed to allow the other parties an active role not only in drafting the program of the new government but also in choosing the Cabinet ministers.

Doubt remained, however, that this concession would satisfy the Communists.

The Communists demand a formal role in the government, not just as part of the parliamentary majority supporting the government.

"All of the democratic forces, including the Communist Party, should be included in the Government," Enrico Berlinguer, the Communist leader said Friday after seeing Pertini. Meanwhile, the official Communist Party newspaper L'Unita accused the Christian Democrats of moving toward early elections with an "anticommunist crusade."

None of the parties favored early elections, however, during the consultations with Pertini and the president appeared determined to try to avoid elections this year. National elections normally would come in 1981.

The Christian Democrats, the largest party but with only 42 percent of the seats in the Chamber of Deputies, moved today to try to find a compromise with the Communists, who, with 36 percent of the chamber, can block most legislation and bring down almost any government.

Andreotti is expected to try to win support for another minority Christian Democratic government by doing what he did just a year ago, getting the Communists, Republicans, Socialists and Social Democrats to form a parliamentary majority with his Christian Democrats to support the government. That formula gave him more than 92 percent of the votes in parliament.

Andreotti had resigned Wednesday after the Communists announced they would no longer support the government in the parliamentary majority formed last March.