CHICAGO -- In the airline industry's latest move into cyberspace, three more U.S. carriers have joined Priceline.com, giving a big boost to the name-your-own-price Internet ticket service.

The endorsement Wednesday by United Airlines, American Airlines and US Airways means that nearly all major domestic airlines now sell seats through the service. The move also doubles the number of seats Priceline can sell.The agreement gives travelers more reason to buy tickets online and provides airlines with a proven way to peddle unsold airline seats at the last minute.

It also deals another blow to travel agents, reeling from the latest round of airline commission cuts as well as new competition from travel Web sites.

Most airlines now sell tickets on their own sites, and last week United, Delta, Continental Airlines and Northwest announced plans for a joint site.

Priceline's three new carriers -- Nos. 1, 2 and 6 in the country -- join Delta Air Lines, Continental Airlines, Northwest Airlines, Trans World Airways and America West Airlines on Priceline. Among major carriers, Southwest Airlines is still not participating.

Priceline's new partners had previously been reluctant to sell tickets there, particularly American, which feared stirring competition with Travelocity.com, the travel Web site controlled by its parent AMR Corp.

But results apparently persuaded them; Priceline says its weekly sales of airline tickets have increased to more than 50,000 a week, up 1,000 percent from January. That's still a tiny fraction of the roughly 10 million tickets sold in the United States each week.

At Priceline.com, buyers can choose their own price for hotel rooms, cars, home mortgages and equity loans. The deal is sealed when the seller accepts the buyer's bid. Wednesday's deal "was just a matter of time," said Ray Neidl, an analyst who follows the airline industry for ING Barings in New York. "Web sites are going to become more commonplace, and airlines -- if there's no cost to them -- will try to take advantage of all the revenue streams possible."