The Philadelphia 76ers and Los Angeles Lakers, favorites to meet in the National Basketball Association's championship series for the second straight year, begin playoff action this afternoon after a week's rest due to first-round byes.

Their debuts will be overshadowed by the deciding third games in miniseries involving the Atlanta Hawks and Boston Celtics in the Eastern Conference and the Phoenix Suns and Denver Nuggets in the Western Conference.

The Hawks travel to Boston Garden in their bid to upset the Celtics (WDVM-TV-9 at 1 p.m.). Atlanta tied the series with a 95-93 victory at the Omni Friday night.

In Phoenix, the Suns, who most likely will be without injured forward Maurice Lucas, will try to elminate the Nuggets (ESPN, 9:50 p.m.). Lucas incurred ligament damage in his left foot in the second game of the series, won by the Nuggets in Denver, 113-99, Thursday night. He is listed as extremely doubtful tonight.

Phoenix Coach John MacLeod isn't counting on having Lucas. He said he will start Alvin Scott and match him against Kiki Vandeweghe.

The San Antonio Spurs are waiting to face the winner. The Milwaukee Bucks will play the Atlanta-Boston winner.

Philadelphia coasted through the regular season with the league's best record (65-17), but the physical condition of Moses Malone, the NBA's top rebounder (15.3) and fifth-leading scorer (24.5), has the 76ers both concerned and vulnerable.

Malone missed the last four games of the regular season with tendinitis in his right knee. After practicing one day, he experienced swelling in his left knee and didn't practice two more days.

But he worked out yesterday, and Coach Billy Cunningham said Malone will start against the New York Knicks in the first game of the best-of-seven Eastern Conference semifinals at the Spectrum.

"Moses is going to start," Cunningham said. "The question is, will he be 100 percent, 90 or 80 percent?"

The Lakers, without injured Bob McAdoo and James Worthy, will begin defense of their title against in the Western Conference semifinals against the Portland Trail Blazers, upset winners over Seattle in the first round. The game will be televised on WDVM-TV-9 at 3:30.

"It's our title, and we aren't going to relinquish it easily," said Coach Pat Riley. "I don't care how many injuries we have." EASTERN CONFERENCE

76ers-Knicks

The Knicks had little trouble sweeping the New Jersey Nets in the first round, and their defense and depth will cause trouble for the 76ers. Bernard King most likely will offset Julius Erving. King had 40 points in the first game of the Nets' series.

The 76ers had two days of two-a-day workouts last week to try to find their lost game and develop a half-court offense. "We're rested and loose," said Erving.

The Knicks beat the 76ers only once during the regular season, but held them to 76 points in doing so. New York led the NBA in defense, yielding 97.5 points a game; the key to the series is to slow the 76ers. WESTERN CONFERENCE

Lakers-Trail Blazers

Portland looked dynamic in ousting Seattle in two straight, and guard Jim Paxson is blossoming into one of the NBA's best all-around guards. Portland beat Los Angeles in the last game of the regular season.

Because of the injuries to McAdoo and Worthy, Magic Johnson will probably have to play power forward more than Riley would like. Worthy is out the rest of the year with a broken leg and McAdoo is recuperating from foot surgery and is doubtful for this series.

No team has repeated as league champion since the Celtics did it in 1968 and 1969.