Time Inc. is now the third largest media company in the nation, replacing RCA Corp., according to a study by Advertising Age magazine.

The survey found that American Broadcasting Cos. remained in first place, followed by CBS Inc.

Time's ascension in the ranks reflects an increase in revenues from media operations in 1982 to $1.8 billion, a 21 percent increase over the previous year's earnings. It was the first time that a company which does not own a network has made it into the top three. RCA owns the National Broadcasting Co. (NBC).

A major factor in increasing Time's media revenues was Time's pay cable network, Home Box Office, and its cable system operator.

The top 10 media companies accounted for 45 percent of the $37.2 billion in media revenues among the 100 companies surveyed. The top 10 companies and their media revenues were: American Broadcasting Cos., $2.53 billion; CBS Inc., $2.45 billion; Time Inc., $1.83 billion; RCA Corp., $1.79 billion; Gannett Co., $1.49 billion; Times Mirror Co., $1.49 billion; S. I. Newhouse & Sons, $1.35 billion; Knight-Ridder, $1.3 billion; Hearst Corp., $1.3 billion; Tribune Co., $1.24 billion.