Three downtown San Diego office buildings, including the distinctive Emerald Plaza tower, have been sold to a Santa Ana real estate firm for a combined $274.5 million.

While the sales prices for each individual high-rise fell short of some other recent sales, the combined transaction is believed to be the largest office-building deal in San Diego history.

Triple Net Properties bought the 30-story Emerald Plaza at 420 W. Broadway, the 24-story Comerica Building at 600 B St., and the 22-story Golden Eagle Plaza at 525 B St.

Southwest Value Partners sold the towers, which the privately held San Diego real estate firm has owned since the mid-1990s.

Southwest declined to say how much it paid originally for the buildings.

“We did very well on the transaction,” said Mark Schlossberg, managing director of the firm. “We felt they were the right buyer, and it was the right price.”

With the purchase, Triple Net becomes the largest landlord in downtown San Diego, with 12.5 percent of the existing office inventory, said Stephen Corea, senior vice president of acquisitions.

“I’ve been around long enough to know that at one point downtown San Diego was derelict and empty,” Corea said. “As far as all the downtowns in the country, I can’t think of one that has covered the kind of distance that San Diego has in terms of improvement.”

Indeed, pension funds and other institutional investors have been gobbling up downtown office towers recently. Among the larger deals have been One America Plaza, which fetched $166.2 million; Symphony Towers, which sold for $134 million; and the First National Bank Building, which garnered $112 million.

“Vacancy in the newer buildings is right at 8 percent, so from an institutional investment standpoint, downtown San Diego is right at the top of the list,” said Kraig Kristofferson, a broker with CB Richard Ellis.

High prices paid by office buyers mean that rents downtown are likely to increase, Kristofferson added.

Real estate experts use price per square foot to measure how much a buyer paid for a high-rise. At Emerald Plaza, which was built in 1990 and ranks among the most recognizable office buildings downtown, Triple Net paid $100.94 million – or $283 per share foot.

That’s just under the $285 per square foot that GE Pension Trust paid for One America Plaza in late 2002.

Golden Eagle and Comerica are older buildings, built in the late ’60s and early ’70s. Triple Net paid $173.5 million for those two towers, or a combined $227 per share foot.

Triple Net, controlled by Anthony Thompson, is the largest so-called tenant-in-common real estate company in the country, with 19 million square feet under management, Corea said.

Tenant in common is a complicated investment vehicle that’s structured as a tax-deferred exchange. It works like this: Triple Net sells a deeded piece of a high-rise building to a pool of individuals, who buy their interest with proceeds from another property sale.

Under tax law, people who sell investment property can defer capital gains taxes if they roll over all the proceeds into similar property.

“It’s for somebody who sold a four-plex or the rental house in La Jolla and has $1.5 million in cash,” Corea said. “They’re able to buy a piece of an institutional-quality office building.

Emerald Plaza will have the tenant-in-common ownership structure, Corea said. The offering, which is treated like a formal securities sale and handled through licensed brokers, is sold out.

The Comerica and Golden Eagle towers will be owned by G Reit, a real estate investment trust that’s a sister company to Triple Net. They will not have tenant-in-common ownership.

Triple Net and its subsidiaries used to focus on buying supermarket-anchored shopping centers. About two years ago, the company began buying office buildings, including towers in Washington state, Texas, Florida, Colorado, Nevada and Hawaii. It also has office holdings in downtown Los Angeles, Sacramento and elsewhere in California. This is its first acquisition in San Diego.

The company has been on a buying binge lately, however. In April, it paid $97 million for Hawthorne Plaza in downtown San Francisco. And in March, Triple Net paid $46 million for the Madrona Building in Torrance, which also was owned by Southwest Value Partners.

Kevin Shannon, Scott Schumacher and Michael Moore of Grubb & Ellis’ office in South Los Angeles worked on the Torrance deal, which paved the way for the San Diego purchases.

“This is Triple Net’s largest acquisition to date,” Shannon said. “They should place over $1 billion into office properties this year, which is major for a tenant-in-common buyer group.”