On a Thomas Circle block where the main business nowadays is done by $50-a-trick prostitutes, three developers plan to build a trio of buildings that will cost more than $80 million.

Two 12-story office buildings and a large office-residential complex are planned on the block along Vermont Avenue south from the circle.

Demolition of the Burlington Hotel at 1120 Vermont Avenue and the vacant parking garage next to it is expected to begin next month. National Content Liquidators of Dayton, Ohio, is selling the hotel's furnishings and fixtures.

Construction is expected to start this summer on the two office buildings. Both are speculative projects, built without advance commitments from major-tenants, said a real estate industry source involved in the deal.

The willingness of builders to put speculative projects in what has been a sordid neighborhood indicates the rapid changes in downtown real estate, said a spokesman for DRG Financial Corp., which arranged financing for all three projects.

"Big boys from out of town and new boys on the block downtown" are moving into what had been Washington's "old downtown," said Michael Lipson of DRG.

DRG Vice President John C. York said the firm arranged financing of $22 million for the building on the Burlington site, $21.5 million loan for the building south of the Burlington and a $7.9 million loan for the vacant land at the north end of the block.

The $51 million to be invested in the three parcels does not include the building planned on the vacant site, which is expected to push the total cost of redoing the block past $80 million.

No details of the office-apartment building on that site have been revealed.

The land recently was sold to International Developers Inc., a group headed by Giuseppe Cecchi. The group has built two major condominium projects in suburban Virginia, Watergate at Landmark and The Rotunda.

A group headed by Washington developer Sylvan Herman will build a 12-story, 478,000-square-foot office building with parking for 582 cars in the basement on the Burlington site. The new building has been designed by Peter Ellenbogen of OKE Inc., Silver Spring.

South of the Burlington, another 12-story office tower, with 295,000 square feet of space and parking for 300 cars is planned by the Stillman Group, a New York City firm. Stillman has developed apartment complexes in Prince Georges County, but the office building will be its first District of Columbia project.

Real estate developers estimate it will take 18 months to demolish the present buildings on the land and build the new ones.