All three major Democratic mayoral candidates reported yesterday that they have thousands of dollars on hand for last-minute campaigns before the Sept. 12 primary, but they also said that their unpaid obligations have placed them heavily in debt.

City Council Chairman Sterling Tucker, who has led the mayoral campaign money-raising derby almost all year, reported collecting $66,257 in the last three weeks of the campaign. But Tucker's campaign, which has spent or incurred obligations totaling $313,073, has an overall deficit of $38,700.

Meanwhile, Mayor Walter E. Washington reported raising $59,621 since the last campaign finance reporting date, Aug. 10, and said he has $19,768 on hand. The mayor's campaign, which has spent or incurred debts totaling $217,506, showed an overall deficit of $27,383.

City Councilman Marion Barry, reported contributions of $46,980 in the last three weeks and a cash balance of $7,197. But like its two rivals, Barry's campaign which has spent $234,214, reported an overall deficit, $5,658.

The latest campaign finance reports, filed with the D.C. Board of Elections and Ethics, show that Tucker continues to receive large contributions from corporate interests in the city, while the mayor has received large amounts of cash from unions and, to a lesser degree than Tucker, from business interests.

Barry continues to receive smaller donations than his rivals and less money from corporate interests.

Several of Tucker's campaign advisers said they were delighted with raising the most money in the last three weeks and said it indicated that last-minute contributors are swinging to Tucker's candidacy in the belief that he represents "the best prospect for change in city government." They said the large debt was typical of any campaign and that it had decreased by $8,300 since Aug. 10.

Tucker has received 23 contributions of $1,000 or more since Aug. 10, compared to 14 for the mayor and three for Barry.

Among Tucker's largest contributors were political action committees representing corporations. Contributions of $1,000 each have been received from Mental Health Inc., Jean's Inc., Rozansky & Kay Construction Co., Queequey Inc., Land Development Institute Ltd., Potomac Distributing Co., National Property Law Digests Inc. and the William Calomiris Investment Co., the Mid-Atlantic Nephrology Center Inc., American Sales Co. Inc., American Health Services Inc. and North American Housing Corp. have contributed $2,000 apiece to the Tucker campaign.

In addition, Tucker received $500 apiece from Elaine Decker Rosenweig, president of the Lewis and Thomas Saltz clothing store, and W. Reid Thompson, the board chairman and president of Potomac Electric Power Co.

The mayor received $15,113 of the $59,621 he raised in the last three weeks from labor unions, with the Marine Engineers Beneficial Association, the Professional Air Traffic Controllers and the International Association of Machinists and Aerospace Workers contributing $2,000 apiece and four other unions pitching in $1,000 apiece.

The mayor also received more contributions from local liquor dealers, many of whom have said they want longer store hours and a city-run lottery to help the industry recover from sagging sales. Eagle Wine & Liquor made the largest contribution among the stores, $1,000, although several others gave $500 or $200.

Tucker, Washington and Barry all reported heavy spending for campaign printing or their media advertising, with Tucker still owing his media consultant, Bailey, Deardourff & Association, a total of $26,500. Washington owes his consultant, Matt Reese & Associates, $20,000.

Another Democratic contender for mayor, Dorothy Maultsby, reported raising a total of $5,431, while the fifth candidate in the party mayoral primary, John L. Ray, reported collecting $12,895, although he showed a debt of $4,926.

City Councilman Arrington Dixon, running for the council chairmanship, reported raising a total of $97,247 and said he has $13,201 on hand. His chief rival, Councilman Douglas E. Moore, said he has received contributions totaling $9,158 and has $3,529 on hand.