SAN FRANCISCO, AUG. 27 -- Joe Montana sent San Francisco ahead with his first touchdown pass of the NFL preseason, a seven-yarder to Mike Wilson just before halftime, and the 49ers went on to their third preseason victory, beating the San Diego Chargers, 17-3, tonight.

Montana completed six of nine attempts for 71 yards as the 49ers drove 81 yards for the touchdown that broke a 3-3 tie. He passed for 134 yards in his one half of action, after completing only five of 16 passes for 46 yards last weekend against Dallas.

Steve Young, the 49ers' new backup quarterback, completed a 44-yard pass to Tony Hill in the fourth quarter to set up rookie Terrence Flagler's three-yard touchdown run.

San Francisco (3-1) registered six quarterback sacks. San Diego (1-2) had only 28 net yards in the first half and did not reach the 100-yard mark until the final period.

San Diego quarterback Dan Fouts, who joined the team late last week, completed only three of 11 passes for 14 yards in his first preseason game. San Diego 3 0 0 0 3 San Francisco 0 10 0 7 17 First Quarter

SD -- FG Abbott 41, 7:33 Second Quarter

SF -- FG Wersching 42, 6:57

SF -- Wilson 7 pass from Montana (Wersching kick), 14:24 Fourth Quarter

SF -- Flagler 3 run (Wersching kick), 9:15

A -- 52,733 San Diego S.F. First downs 12 23 Rushes-yards 25-49 33-212 Passing-yards 67 183 Return yards 28 27 Passing 18-38-2 20-36-0 Sacked-yards lost 5-62 6-34 Punts-average 11-38 8-36 Fumbles-lost 0-0 3-2 Penalties-yards 6-45 9-53 Time of possession 29:4

RUSHING -- San Diego: Redden 15-16, Holland 1-9, Tim Spencer 2-8, Todd Spencer 1-6, James 2-6, Adams 3-5, A

PASSING -- San Diego: Vlasix 8-14-1, 70 yards, Flick 7-13-1, 45, Fouts 3-11-0, 14. San Francisco: Montana 1

RECEIVING -- San Diego: Ware 4-50, James 3-23, Anderson 2-20, Winslow 2-12, Adams 4-9, Tim Spencer 2-6, Red

MISSED FIELD GOALS -- San Francisco, Wersching 34.