Three of the Washington area's leading real estate investment trusts reported increased earnings, but one -- B.F. Saul -- reported a significant drop.

B.F. Saul Real Estate Investment Trust reported a net loss of $352,000 compared with a net income of $4.6 million (78 cents a share) for its third quarter ended June 30.

Gains on the sale of properties other than condominiums amounted to $42,000 in the 1981 period compared with $169,000 for the same time last year.

Cash flow from operations was $1.2 million compared with $5.7 million. The decrease was due primarily to a $5.6 million reduction in condominium profits caused by rising home mortgage interest rates and subsequent lower demand for units.

For the nine months ended June 30, net income was $4.9 million (82 cents) compared with $5.1 million (87 cents) a year earlier.

Cash flow from operations for the nine months was $10.7 million compared with $8.5 million. That increase was due to "a continuing improvement in tbe performance of the trust's commercial and residential properties," the company said.

Washington Real Estate Investment Trust reported a 30 percent increase in second-quarter earnings from $899,947 (20 cents a share) to $1.2 million (25 cents). Total income was $3.2 million compared with $3.0 million. First-half net income was $2.1 million (44 cents) compared with $1.6 million (36 cents). Total income was $6.5 million compared with $6.1 million.

Federal Realty Investment Trust reported an increase in net income for the second quarter of $1.1 million (52 cents a share, fully diluted) compared with $322,000 (21 cents). Revenues were $3.5 million, up from $2.5 million.

For the first six months, net income was $1.5 million (76 cents) compared with $803,000 (53 cents). Revenues were $7.1 million, up 42 percent from $5.0 million.

"The trust's 19 shopping centers continued their strong performance with rental rates escalating due to a strong demand for retail space," the company said. "Percentage rental income, geared to retail tenants' sales volumes, was 41 percent ahead of the level for the earlier comparable period. Assisted by a tightening metropolitan Washington rental market, cash flow from apartments is also well ahead of past years' levels."

Riviere Realty Trust reported net income for the second quarter of $47,714 (6 cents a share) compared with a loss of $246,638. The firm also generated funds from operations of $207,458 compared with $96,617.

First-half net income was $151,592 (19 cents) compared with $350,860 (44 cents). Funds from operations and capital gains totaled $468,861 compared with $642,193.