A motorist was killed and two were injured at the start of yesterday's afternoon rush hour when three vehicles collided on the Capital Beltway a half mile east of Connecticut Avenue in Montgomery County, police said.

A major traffic jam ensued, with the Beltway's eastbound lanes remaining closed for almost two hours between Connecticut and Georgia avenues. Trooper Kenneth Oland of the Maryland State Police said the backup extended at least as far as Old Georgetown Road about three miles to the west.

The accident occurred about 3:15 p.m. when a westbound car veered across the median and hit a pickup truck in the Beltway's eastbound lanes, police said. Seconds later, a station wagon hit the truck, according to police.

The driver of the car, Seymour Levinson, 66, of 9619 Alta Vista Ter., Bethesda, was killed.

The truck's driver, Harley Leo Shirley, 22, of 3521 Queenstown Dr., Mount Rainier, was reported in critical condition last night at Suburban Hospital. The driver of the station wagon, Gail Middlekauff, 49, of 6013 Overlea Rd., Bethesda, was treated at Suburban Hospital for what were described as minor injuries and released, a hospital spokesman said.

Oland said none of the drivers appeared to have been wearing a seat belt. Police 1st Sgt. Douglas Slacum said Levinson's sedan and the pickup truck were demolished in the collision. He said police had not determined why Levinson's car crossed the median.