Three top Democratic Party officials yesterday urged the three major television networks to reject President Reagan's request for free air time to speak about the economy Wednesday night to avoid "a precedent of network complicity in last-minute partisan campaigning by the president."

NBC will give the president a half hour of free air time at 7:30 p.m. for his speech on the economy, a Democratic spokesman said. ABC and CBS as yet have made no decision, the spokesman said.

The president made the request last week after federal statistics showed that the national unemployment rate has risen to 10.1 percent.

In telegrams to the presidents of all three networks, the Democratic officials recalled that in 1978 the networks rejected a request by President Carter to address the nation on July 4, when the election was four months away. Carter's request was rejected on the grounds that his address would not be "newsworthy."

Now, the Democrats contended, this same standard should be applied to Reagan, who has requested air time just 20 days before an election--in the midst of his own campaign travels in behalf of Republican candidates.

The telegram was signed by Democratic National Committee Chairman Charles Manatt, and the chairmen of the Senate and House Democratic campaign committees, Sen. Wendell Ford (D-Ky.) and Rep. Tony Coehlo (D-Calif.).

NBC has promised to give the Democrats a half hour of air time the day after the president speaks, according to one Democratic spokesman. But the Democrats say that they ought to be allowed to respond to the president immediately after Reagan speaks, if indeed he is allowed the air time.

That would create problems for NBC since it intends to carry the World Series immediately following the president's speech.

Manatt also urged all Democrats running for the Senate and House to request separately that they be given equal time to respond to Reagan's address, on the grounds that it constitutes a free campaign message that is being aired to the voters in their states. The Democrats argue that each candidate should be allowed the time to respond under the Federal Communications Commission's fairness doctrine.