CHICAGO — The young Chicago Cubs learned a pointed lesson on establishing a killer instinct on Saturday.

Leading 5-3 heading into the bottom of the eighth against the San Francisco Giants, Chicago added three runs for a seemingly insurmountable 8-3 edge.

But it wasn't a comfortable at all. The Giants replied with three of their own before the Cubs finally locked down an 8-6 victory.

"I'm still looking for that late inning swagger and I think we're almost there where if things get a little freaky were still OK. That's the next step," said Cubs manager Joe Maddon.

The victory gave the Cubs their first series victory over the Giants at Wrigley Field in seven seasons and put Chicago in position for a four-game sweep on Sunday.

Shortstop Addison Russell went 3-for-4 and third baseman Kris Bryant hit a two-run homer as the Cubs (61-48) won their third straight in the four-game series, which concludes Sunday.

San Francisco, which had a 4-0-3 edge in seven previous series in Chicago since 2008, fell 2 1/2 games behind the Cubs in the chase for the National League's second wild card playoff spot.

Right-handed starter Matt Cain (2-3) lasted just four-plus innings as the Giants (59-51) dropped their third straight.

Chicago right-hander Kyle Hendricks (6-5) worked 5 1/3 innings for the win as the Cubs claimed their ninth victory in 10 games.

"I went out after we scored runs today twice and gave up a couple -- which you don't like doing," said Hendricks. "But the confidence in that lineup is high right now. We knew we were going to score some runs and turn it around."

Cubs right-hander Justin Grimm entered with two outs in the ninth to claim his third save of the season.

"They did a good job fighting, coming back at the end and even early in the game," Giants manager Bruce Bochy said. "Here they came back in the ninth and we got the tying run up there. It was a pretty impressive job."

Bryant, the Cubs' third baseman, was 1-for-3 with a walk, two runs scored and three RBIs. Russell, the Cubs' shortstop, had two doubles, plus an RBI single in the eighth.

The Cubs opened a first-inning lead for the third straight game as they loaded the bases with one out on Cain. Bryant beat out a fielder's choice to drive in left fielder Kyle Schwarber for a 1-0 edge.

The lead was short-lived as Giants first baseman Brandon Belt collected his 15th home run of the season in the second. Belt's two-run shot off Hendricks landed in the right-field basket and brought home right fielder Hunter Pence.

In the third, Bryant launched his 15th home run of the season as the Cubs jumped back ahead 3-2. Bryant sent Cain's 0-2 pitch to left with one out to score first baseman Anthony Rizzo.

San Francisco forced a 3-3 tie in the fourth when Pence scored from first on shortstop Brandon Crawford's double to deep left.

Cain departed with none out in the fifth after walking Bryant and giving up a hit to right fielder Jorge Soler. Cain gave up five runs on six hits and five walks, striking out two.

"Matty worked hard today," Bochy said. "Pitch count got up there, he was in the stretch every inning and couldn't get a one-two-three inning and that caught up with him."

Right-handed reliever George Kontos gave up a run-scoring single to catcher Miguel Montero, who was playing his first game since coming off the disabled list.

Montero's hit scored Bryant, and Hendricks grounded into a double play that scored Soler from third, giving the Cubs a 5-3 lead.

Hendricks got a groundout to open the sixth but then allowed two runners and departed. Left-hander Travis Wood retired the next two batters to quell the threat.

Hendricks worked 5 1/3 innings and gave up three runs on five hits, walking two and striking out five.

NOTES: San Francisco entered the game among the National League's top offensive teams, ranking second in runs scored (479), batting average (.272) and on-base percentage (.328). ... With seven weeks left in the season, Giants SS Brandon Crawford already has career highs in home runs (19) and RBIs (71). ... San Francisco RHP Jake Peavy (2-4, 3.77 ERA) will go against Chicago RHP Jake Arrieta (12-6, 2.50) in the series finale Sunday. ... The Cubs placed C David Ross on the bereavement list and called up OF Matt Szczur from Triple-A Iowa. Szczur joined the Cubs for the sixth time this season. ... Saturday marked the 27th anniversary of the first scheduled night game at Wrigley Field. The game was rained out in the bottom of the fourth and the first official night game was played on Aug. 9.