The Philadelphia 76ers are ready to drop the guillotine on the helpless New York Knicks today as they dance into the fourth game of their best-of-seven National Basketball Association Eastern Conference playoff semifinal series.

The confident 76ers humiliated the outclassed and confused knicks by 40 and 19 points, respectively, in the first two games of the series in Philadelphia, and then took a three game lead with a wild 137-126 victory at Madison Square Garden Thursday night. The Knicks played about as well as they can in that game and they still were virtually in over their heads.

"If we're playing our best ball, there's no way we're going to win." said New York forward Jim McMillian.

"I don't know what we can do to heat them," added forward Lonnie Shelton, "Maybe we can't. I guess it boils down to them being a better team than us."

In the Western Conference playoffs today, the surprising Seattle Super-Sonics host the descending NBA champion Portland Trail Blazers with that series tied at 1-1.

The Trail Blazers suffered a stunning blow yesterday when the club announced that center Bill Walton will be lost for the remainder of the playoffs because of an ankle fracture. Walton, the NBA's most valuable player this season, sat out the second half of Portland's victory over Seattle Friday night. Later, he was placed in a walking cast when doctors discovered the fracture in his left ankle.

The Sonics had upset the Trail Blazers, 104-95 in the opener in Portland Tuesday and with the next two games at the Seattle Coliseum, and with Walton out, Seattle is in a good position to knock off the champs.

Center Marvin Webster has emerged as the most dominant big man in the playoffs. He erased Kareen Lakers in the first round of the playoff, and with Walton out, is doing the same to Portland.

The Denver-Milwaukee series goes to Milwaukee today with the Nuggets leading two games to none. They had little trouble from the young Bucks in the first two games in Denver, but the Nuggets have a history of not doing very well in playoff games away from home.

The Knicks are the only team with practically no chance of advancing.

As Coach Willis Reed said, "We showed in the series with the 76ers.

Bob McAdoo, Spencer Haywood and Shelton are all natural forwards, yet one of them has to play center, and none does it particularly well. But that is only part of the Knicks' problem.

The 76ers have dominated the Knicks in every way conceivable. They have averaged 129 points in the three games and have won them by a total of 70 points. Philadelphia has also shot 38 more free throws.

The 76ers are so much better than the Knicks that the Philadelphia bench has outscored the New York starters in two of the games.

"We one on one them to death," said Philadelphia's George McGinnis.

There's more to it than that, according to New York's Phil Jackson.

"The times we beat them in the past, they didn't play as a unit," said Jackson. "They went one on one. Now they are more of a team than I have ever seen them. They are the best at going from defense to offense. As soon as they smell the scent of the ball, two guys are off and running."