Two field goals by Julius Erving in the final 56 seconds, after Philadelphia had blown an 18-point lead, gave the 76ers a 109-104 victory over the Milwaukee Bucks last night in Philadelphia. Philadelphia leads the best-of-seven NBA Eastern Conference semifinal series, 3-0.

The Bucks led, 104-102, with 1:20 to play after breaking the seventh tie of the fourth quarter on a basket by Alton Lister. Erving then hit a jumper from about a foot inside the three-point line as the 24-second shot clock was about to run out, tying the score with 56 seconds left.

"It was a big one," Erving said of his basket, "because it put a lot of pressure on them. I knew it was good when I saw it coming down.

"Then we came back and were able to force a turnover that gave us the breakaway basket."

Maurice Cheeks stole the ball from Terry Cummings after the Bucks had brought it downcourt and passed to Erving for a layup that put the 76ers ahead to stay at 106-104 with 38 seconds remaining.

Milwaukee had two more chances to tie, but jump shots by Lister and Ricky Pierce failed to fall.

"This was more like I expected the series to be, a truly defensive battle," said 76ers Coach Billy Cunningham. "It was nice to see our defense rise up at the end and create some opportunities for us to win."

Andrew Toney led the 76ers with 20 points. Erving and rookie Charles Barkley each had 19 for Philadelphia, which has a chance to finish a 4-0 sweep here Sunday in Game 4.

Cummings had 21 points, 17 in the first half, and Paul Pressey scored 20 and Sidney Moncrief 17 for the Bucks.

The Bucks trailed, 85-77, at the start of the final period, but scored the first eight points of the quarter to tie with 9:53 left.

The score was tied five more times, the last at 97-97, before a field goal by Cummings at 2:48 sent Milwaukee ahead, 100-99, for the Bucks' first lead since the first quarter. Pierce hit two free throws for a 102-99 edge.

But Barkley made a three-point play at 1:35 to tie it again at 102. Lister's field goal gave the Bucks their last lead. Milwaukee didn't score again.