D.C. police said yesterday that the latest rash of heroin overdoses in Washington -- three deaths and six other known overdoses since Monday -- does not appear to result from an unusually potent or contaminated batch of drugs, but does indicate the growing extent of the heroin problem here.

Inspector Kris Coligan, head of the morals division, said police believe the heroin that caused the overdoses Monday evening and early yesterday was bought from several different suppliers. He said samples tested so far seemed to be below average in purity.

Coligan said this is the largest group of drug overdoses since the nine drug deaths in March. Those earlier deaths were deemed related, but this week's apparently are not, Coligan said.

"There have been other instances when we have had three overdose deaths in one day," said Coligan. "What this tells me is that we have a terrible problem with drug abuse, a terrible problem with the availability and supply of heroin, and a number of people willing to take the risk of dying to buy their heroin and shoot up."

Police said that this year to date, there have been 71 fatal overdoses in Washington. Last year at this time, there were 50. The total number of fatal drug overdoses for 1984 was 140.

Police have identified two of the three men who died this week as Percy Watts, 47, of 601 Four Mile Rd., Alexandria, and Mark S. Duncan, 35, of 5783 Suitland Rd., Suitland. The third man, who was found in a hallway at 1430 W St. NW, is unidentified, police said.

Coligan said that a packet of heroin recovered from one of the six known overdose victims who survived was found to contain 4 percent pure heroin, which is considered low in purity. Coligan said that heroin does not have to be high in purity to kill. The victim could have a low tolerance for drugs or could have also used other drugs that created a lethal combination, he said.

Police said two men were arrested in the 1300 block of Florida Avenue NW about 2:15 a.m. Tuesday and charged with selling heroin, but investigators had not found a connection between the heroin seized from the men and the overdose deaths.

Other sources said police believe the drugs bought by the recent victims came from two well-known heroin markets in Northwest Washington: Eighth and N streets and 14th and W Streets. Street dealers give their wares colorful names to create a kind of brand identification; one of the "brands" that caused a fatal overdose was called "Maniac," a source said.

Detective Al McMaster, a narcotics investigator assigned to the cases, said the most difficult part of investigating drug cases is the "rumor mill." He said, "We get thousands of calls in situations like this where everyone thinks they know who sold the dope. We have to check out each and every tip and that takes a lot of time." He said drug dealers sometimes call with tips in an attempt to rid themselves of competitors.