The Washington Diplomats, having virtually assured themselves of the home-field advantage for the first round of the playoffs, began preparing for their final three regular-season games yesterday, games they will play without their captain, and, in all likelihood without their all-time leading scorer.

Team captain Jim Steele will miss the final three regular-season games, including the final home game Sunday at RFK Stadium against Toronto at 2:30 p.m. He is serving a three game suspension handed down by the North American Soccer League.

Steele was suspended automatically for one game because he was ejected from Wednesday's 5-1 Washington win over Tampa Bay. Two games were tacked on because the 10 penalty point he received through the ejection gave him 35 this season. That calls for an automatic two-game suspension.

The club's all-time leading scorer, Paul Cannell, definitely will not play Sunday and might not play again soon for a different reason: Coach Gordon Bradley has benched him for poor play.

Cannell said Wednesday, after Bobby Stokes had replaced him at striker and scored three goals, that he thought he had played his last game with the Diplomats. Cannell said he wants to be traded at the end of then season.

"If he wants to be traded, that's fine," Bradley said. "But he'll be traded when I want to make a deal, when I'm ready. Paul's just like any other member of this team. I'm the coach and I make the decisions on trades."

Cannell was supposed tp practice with the reserves yesterday morning but did not show up. Bradley said Cannell was not at fault. "I wanted him here but I didn't tell him specifically he had to be here," Bradley said.

The muddled National Conference playoff picture became even more confusing following Wednesday night's games. Only one thing is certain: The Cosmos, 21-6 for 179 points, will be the top seed in the division and will play the last- -or eighth place- -qualifier in the first round. Right now that would be Toronto.

Minnestoa, Washington and Vancouver are battling for second place. All have three games left and Minnesota has 166 points, Washington 160 and Vancouver 157. The No. 2 team will gain the home field edge for at least two rounds.