Despite the opening of three new Red Line stations in Northwest Washington, Metrorail ridership declined slightly in April from the same month last year. Metro officials attributed the unprecedented drop to cheaper gasoline, high unemployment, a fare increase and the fact that more trains are breaking down at rush hour.

Bus ridership also did poorly. Metro planners had expected it to fall by about 6,000 rides a day as parallel bus service was cut back around the new Metro stations. But in March, it had declined by 13,000 rides from the previous March, and in April 20,000 from the previous April.

The ridership report, released yesterday at a Metro board meeting, was troubling news for the transit authority, which had expected that running trains to Van Ness/UDC would bolster its stagnated ridership. By lowering revenues, the decrease in subway ridership willexacerbate Metro's financial troubles.

Gasoline prices, fare rises and unemployment have been blamed for past rail ridership problems. Yesterday, General Manager Richard Page pointed to another culprit: poor maintenance. Unusual numbers of suspended trips, many caused by smoking brakes, have cut into the image of speed and reliability that is the subway system's major attraction, officials believe.

In the first months of 1981, Metrorail was providing about 280,000 rides on an average weekday. Planners had expected that for the same period this year the new stations and growth at other stations would push total rail ridership up by about 10 percent, or 27,000 rides a day. They expected this to occur despite a fare increase last December.

But actual rail ridership in the months after the Van Ness opening only slightly exceeded figures for those months in 1981. In April, it dipped below the 1981 figures, losing about 1,700 rides daily, a 0.6 percent drop. It was the first time that rail ridership has ever dropped from the same month the year before.

Metro's budget for fiscal 1983 counts on $10 million in new revenues due to increased ridership. Planners have not yet calculated the impact on revenues the poor showing will have, but some feel that it may force reductions in service.

In explaining the loss, Metro officials acknowledge that rail service disruptions have increased markedly in the last six months. Page said this is due both to mechanical failures and a "play it safe" policy dating from the Jan. 13 derailment that killed three people. Trains are now unloaded at the slightest sign of a problem, he said.

Metro's statistics show that in fiscal 1981 about 1.5 percent of scheduled trips were not completed. Starting in December, when it was 2 percent, the figure has been consistently higher: 4 percent in January, 2.6 percent in February and 3.2 percent in March.

The problems are also reflected in Metro's accounting of trains delayed five minutes or more. The fiscal 1981 average was 0.83 percent. January's figure was 1.3 percent, February's 1.0 percent, March's 0.91 percent.

Metro officials also blame the decline on the December fare increase and the drop in gasoline prices in recent months, which make it more attractive to drive to work. Gasoline cost an average of $1.41 in mid-December last year, according to the American Automobile Association, but had fallen to $1.30 in mid-May.

The ongoing recession and unemployment is also seen as a factor, because a major portion of Metro's riders are going to and from work.

In other developments yesterday, the Metro board:

* Approved reductions in bus service on some routes within Montgomery County and on routes linking the county and the District. Aimed at trips said to have low ridership, the cuts will affect the J2, J4, L8, N4, T1, T2, T4, T6, Y7, Y8, Z2 and Z4 routes.

* Was informed that Metrorail will operate at near rush-hour levels between 6 p.m. and midnight July 4 to take people to and from the fireworks display on the Mall. Fares will be a flat 50 cents all day on bus and rail.

* Approved a report calling for service reliability to be Metro's foremost goal. The move is an effort to end Metro's longstanding preoccupation with rail construction.