By SAM GOLDAPER

To set the stage for the third major trade in the last two weeks, the Bullets, with the sixth pick in the draft, selected Mel Turpin, the 6-foot-11- inch University of Kentucky center and then traded him to the Cavaliers for Robinson and Tim McCormick, the 6-11 University of Michigan center drafted by Cleveland on the 12th pick. The Bullets then packaged McCormick and Ricky Sobers, a starting guard, and sent them to Seattle for Williams, a 30-year-old guard in his ninth pro season.

In still another deal, the Bullets obtained Tom Sewell, a 6-5 swing man, from the Philadelphia 76ers for Washington's top choice in the 1988 draft. Sewell, who averaged 22.9 points a game for Lamar, was the third of the 76ers' three first-round choices, the 22d player taken over all. "We've got enough beef," said Bob Ferry, the Bullets' general manager, explaining why he drafted then gave up the 245-pound Turpin. "I really liked Turpin a lot but with Jeff Rulland and Rick Mahorn, we felt we needed speed and quickness."

Coach Gene Shue of the Bullets, added: "We now have a chance to compete with the better teams."

In Williams and Robinson, the Bullets, who finished 21st in scoring last season, obtained players who averaged more than 36 points a game combined last season. Williams, one of the quickest guards in the league, has a 17.7 career scoring average.

The 6-9 Robinson, who began his pro career as the Nets' first draft pick in 1979, is a very talented scorer and rebounder whose career has been inconsistent at times.

The 31-year-old Sobers, out of DeWitt Clinton High School in the Bronx and Nevada-Las Vegas, also has good backcourt speed. The Sonics will be his fifth pro team. He began as the Phoenix Suns' top draft choice in 1975 and also played for the Indiana Pacers and the Chicago Bulls, before signing with the Bullets last season as a free agent. He averaged 15.6 points last season.

In another trade, the Golden State Warriors, who did not have a first- round selection, traded 6-10 Jay Murphy of Boston College, one of their four second-round choices, to the Los Angeles Clippers, The Warriors received Jerome Whitehead, who has backed up the often injured Bill Walton at center in recent years. Murphy had been the second player the Warriors drafted. They used their first one, the 30th pick, for Steve Burtt of Iona.

On the opening round, each team had 5 minutes to make its selection, but Ray Patterson, the Rockets' president and general manager, used only 39 seconds to announce, "Houston takes Akeem Olajuwon of the University of Houston."

The 7-foot Olajuwon, the nation's top rebounder, wearing a black suit, a tuxedo shirt and a red bow tie, then stepped to the podium, where Charley Thomas, the Rockets' owner, and his daughter, Nancy, presented him with a Rocket uniform number 34.

"Now I know I'm going to Houston," said Olajawon, a soccer player in Nigeria, who went to the University of Houston three years ago to make it big in basketball.

Among those in the audience of about 3,000 at Madison Square Garden's Felt Forum, where the draft was conducted through a telephone hookup with the league's 23 cities, were Olajawon's mother, Abike, clad in Nigerian dress, and his three brothers, Akin, Taju and Yemi.

"My father couldn't come," Olajawon said. "He didn't have a passport."

By teaming Olajwon with the 7-4 Ralph Sampson, the first player taken in the 1983 draft, the Rockets have drawn a blue print for the most awesome center-forward combination since Wilt Chamberlain, 7-1, and Nate Thurmond, 6-11, played together with the San Francisco Warriors for a season and a half starting in 1963.

The first seven selections yesterday held to form. After Portland went for the 7-1 Bowie, Chicago chose Michael Jordan of North Carolina, the college player of the year. The next three picks, by Dallas, the 76ers and Washington, were all strong forwards and centers: 6-9 Sam Perkins of North Carolina, 6-6 Charles Barkley of Auburn and Turpin, respectively. Alvin Robertson of Arkansas was the first guard selected, by San Antonio.

Underclassmen Chosen

Of the nine underclassmen in the draft, three - Olajuwon, Jordan and Barkley - were among the first five players taken. . . . When the Nets made Jeff Turner of Vanderbilt their first pick, the choice was greeted by boos. . . . Michael Young, a 6-7 forward and Olajuwon's teammate, was expected to go much higher than his selection of No. 24 by the Boston Celtics. . . . The Lakers gambled with the 23d pick and took Earl Jones, the 6-10 forward from District of Columbia University, a Division II school. . . . A total of 228 players were drafted during the 10 rounds that took 6 hours 17 minutes to complete. The Lakers were the only team to pass. . . . The 208th player selected, by Chicago on the 10th round, never played college basketball but no one questions his speed or jumping ability. On the day he was drafted as a guard, Carl Lewis was busy competing in the 200-meter dash qualifying rounds and the long- jump final in the United States Olympic trials in Los Angeles. Lewis, who never played football, was also drafted by the Dallas Cowboys of the National Football League. . . . The Bulls, using the second of their two second-round picks, took Greg Wiltjer of the University of Victoria, a center for the Canadian national team.