Three years ago, Joe Green and his wife Rose lived with their five children in an efficiency apartment where they faced the threat of displacement because their landlord was selling the building.

Today, the Greens are coowners of the same building, a five-unit structure at 1312 Euclid St. NW, and are planning to move into a spacious, carpeted four-bedroom apartment next Wednesday.

The Greens were saved from joining the thousands of low-income families displaced in the District by the combined efforts of the Southern Columbia Heights Tenants Union (SCHTU) and the Metropolitan Washington Planning and Housing Association.

The two organizations helped them and the only other remaining tenant secure loans to buy their building and to convert it to a tenant-owned cooperative. They have named it the "Long Time Coming Tenant Cooperative."

Green was one of the speakers Friday, which was called National Tenants Day by the National Tenansts Union, when the SCHTU and the NTU held a news conference to celebrate the tenants' triumph at the Greens' building and to denounce the housing policies of the Reagan administration.

"There's no adjective to describe how I feel," said Green, 28, who has lived in the building all his life and works as athletic director of the Kingman Boys and Girls Club. "A lot of times, even with the loan money, it looked like it wasn't going to happen. Now I'm looking at the building and I still don't believe it."

The Greens and Juanita Jones, the only other tenant who persevered, bought the building in 1981 for $75,000 with a loan from the D.C. Department of Housing and Community Development, according to the tenants' lawyer, Richard Eisen. They obtained long-term financing through a $244,000 federal Section 8 loan from the D.C. Housing Finance Agency and $109,000 from a group of private investors who will receive tax write-offs from the project, Eisen said.

The NTU and its member groups have organized events around the country to push for a congressionally declared annual National Tenants Day. The D.C. City Council proclaimed last Friday as D.C. Tenants Day.

Green and other speakers emphasized that it would be impossible for the Euclid Street tenants to buy their building today because of President Reagan's budget cuts.

A similar project organized by the SCHTU nearby at 2618, 2620 and 2622 13th St. NW failed recently because the tenants were unable to obtain federal loans to renovate the building. The tenants were forced to put the buildings up for sale, spokesmen for the organization said.

"Apparently President Reagan doesn't like the idea of poor people getting control over their homes and lives," said Pat Beynum, a spokeswoman for the SCHTU.

"Unfortunately, with the man in the White House and the type of Congress we've got, it's impossible to purchase a building such as the Greens' . . . . It's a very, very difficult process, and it's getting worse because of the federal housing policy," Eisen said.

Throughout the day, members of the NTU and SCHTU repeatedly attacked the administration's housing policies as designed to penalize the poor and assist the wealthy.

At the news conference later in front of the Euclid Street building, Woody Widrow, a staff member of NTU, announced that the American dream of every family owning its own home "has gone sour."

"It's not the American dream that between two and three million people are homeless. . . .It's not the American dream that we lose 500,000 low-income rental units every year to demolition, conversion, abandonment and arson," Widrow said.

Tenants in Columbia Heights feel increasingly uneasy about the future, according to SCHTU member Beynum. She quoted Census records that show a decrease in the low-income black population of the area, while the white population has increased. Poor tenants who once shared the area's large Victorian houses are being evicted, and the houses are being converted back into single-family dwellings or expensive condominiums, she said.

SCHTU is involved in more than 40 buildings in the area, assisting tenants to press for improved living conditions, repairs to the often dilapidated buildings and rent refunds for tenants who can prove that major housing code violations exist.

"The landlords take the rent but won't make repairs. When conditions become intolerable, people move out. In a passive way, they are being forced out," said Jim Tamialis, an organizer for SCHTU.

The tenants union is giving the residents the courage to fight to stay in the area, Beynum said.

She ended her speech on an optimistic note: "Columbia Heights belongs to us and not the real estate speculators, developers, bankers and landlords. This is our neighborhood, not theirs. . . . And we will fight to stay here."