Rookie shortstop Ed Romero had three singles and drove in the decisive run today in leading the Milwaukee Brewers to a 3-2 victory over the Baltimore Orioles and giving rookie Moose Haas his sixth win.

Haas, who had won only one game since May 25, struck out seven before leaving in the ninth after walking his second batter of the game. Bob McClure couldn't get a batter out and needed relief help himself from Bill Castro, who gave up a sacrifice fly to Eddie Murray and a run-scoring single to Lee May before cutting the rally short.

The Brewers' victory snapped a four-game Orioles winning streak. Baltimore had won 14 of its 16 previous games, but remained a half-game in front in the American League East as both the Yankees and Red Sox lost.

Milwaukee's first run off loser Dennis Martinez came in the first inning when Bob Sheldon led off with a double, went to third on a fly out and scored on Cecil Copper's sacrifice fly. The Brewers added two runs in the eighth on a single by Sixto Lezeano a stolen base and RBI singles by Jamie Quirk and Romero.

Milwaukee had 12 hits, including three by Romero, who was called up from Holyoke, Mass., Saturday to fill in at shortstop for injured Robin Yount.

The Brewers also are written second baseman Don Money, who will miss the All-Star Game Tuesday because of a back ailment. Reserve infielder Tim Johnson is on the disabled list with a groin injury.

Romero, 19, was hitting 227 at Class AA Holyoke and batted 219 for Burlington in 1976, his first professional season.