* “Avatar” grosses $1.02 billion at worldwide box office

* Already fourth-biggest movie of all time

* Poised to beat “Dark Knight” in North America

(updates with worldwide sales, more films in North America)

LOS ANGELES, Jan 3 (Reuters) - “Avatar” sped past the $1 billion mark at the worldwide box office after three weekends in release, making it the fourth-biggest movie of all time, according to data released on Sunday.

James Cameron’s 3-D sci-fi epic earned $1.02 billion, powered by sales of $202 million during the New Year holiday weekend, distributor 20th Century Fox said.

The only movies ahead of it are Cameron’s “Titanic” ($1.8 billion), “The Lord of the Rings: The Return of the King” ($1.12 billion), and “Pirates of the Caribbean: Dead Man’s Chest” ($1.07 billion).

North American moviegoers chipped in $68.3 million, setting a record for a film in its third weekend. The old mark of $45 million was set by “Spider-Man” in 2002.

After 17 days in cinemas across the United States and Canada, “Avatar” has earned $352.1 million, placing it at No. 15 among all movies in terms of ticket sales.

Fox, a unit of News Corp. NWSA.O, forecast the film would end up in the $500 million range in the North American market.

Cameron’s 1997 drama “Titanic,” the biggest film of all time before accounting for inflation, earned $601 million, followed by 2008’s “The Dark Knight” with $533 million. “Star Wars” currently holds the No. 3 berth with $461 million.

“This is like a freight train out of control,” said Fox distribution executive Bert Livingston. “It just keeps on going.”

Even “Titanic” could be threatened. When ticket-price inflation is included, that film had earned $241 million after 17 days, according to tracking firm Box Office Mojo.

FOREIGN SALES AT $670 MILLION

The international total for “Avatar” stands at $670.2 million. The film is playing virtually everywhere. It opened in China on Saturday, and will reach Italy -- its final market -- on Jan. 15.

“Avatar” is the tale of a disabled ex-Marine sent from Earth to infiltrate a race of 10-foot blue aliens and persuade them to let his employer mine their homeland for natural resources. It was reportedly the most expensive film ever made, with a budget of at least $300 million.

In the absence of any major new releases, the rankings of the rest of the films in the North American top 10 were largely unchanged from the previous week.

"Sherlock Holmes" was No. 2 with $38.4 million, taking the 10-day total for the Robert Downey Jr. action-adventure to $140.7 million. It was released by Warner Bros. Pictures, a unit of Time Warner Inc. TWX.N.

Fox’s family offering “Alvin and the Chipmunks: The Squeakquel” followed at No. 3 with $36.6 million, and a 12-day total of $157.3 million.

The Universal Pictures comedy "It's Complicated" held steady at No. 4 in its second weekend with $18.7 million. The film, starring Meryl Streep, Alec Baldwin and Steve Martin, has earned $59.1 million. Universal is a unit of General Electric Co. GE.N.

The Sandra Bullock smash “The Blind Side” was No. 5 with $12.7 million. Warner Bros.’ football-themed drama has earned $209.1 million after seven weekends. (Reporting by Dean Goodman and Jim Finkle; Editing by Paul Simao)