Acting for the second time in three days on rules coverning class-action lawsuits, the Supreme Court decided 9 to 0 yesterday that an order by a federal judge denying certification of a class cannot be appealed.

With Justice John Paul Stevens writing the opinions, the court affirmed a decision by the 3d U.S. Circuit Court of Appeals and reversed a conflicting opinions by the 8th Circuit.

The 3d Circuit held that Jo Ann Evans Gardner, who had been turned down for a job as a talk-show host by a radio station in Pittsburgh, could not appeal the dismissal of her motion to file a civil rights suit in behalf of all women claimed to be victims of the station's allegedly discriminatory policy.

The 8th Circuit acted in a case in which the accounting firm of Coopers & Lybrand certified financial statements in a prospectus issued in connection with a 1972 public offering of securities in Punta Gorda Isles for an aggregate price exceeding $18 million.

Relying on the prospectus, Cecil and Dorothy Livesay of St. Louis bought some of the securities. In the next annual to report to shareholders, Punta Gorda restated the earnings for 1970 and 1971 as reported in the porspectus, writing down net income for each year by more than $1 million.

The couple then sold their shares, sustaining a loss of $2,650. Claiming violations of the Securities and Exchange Acts of 1933 and 1934, they field a class action in behalf of all purchasers similar to themselves. The trial court decertified the class action but was reversed.

The 8th Circuit said that Livesays, whose annual income was $26,000 and who had a net worth of $75,000, only $4,000 of it in cash, could not afford to pursue alone a suit that had cost them $1,200 by December 1974 and promised to cost at least $15,000 more.

In the opinion reversing the appeals court, Justice Stevens said that "appeals of right" from non-final orders that turn on the facts of a particular case can't be allowed because appelate courts then would be thrust "indiscriminately into the trial process . . ."

In a decision Monday, the Supreme Court made it less easy to bring class actions by ruling that plaintiffs usually must bear the costs of compiling the names and addresses of members of the affected class. Often the numbers in a single case may be in the tens of thousands.