Belt connected for his 12th home run this season and fifth in seven games to give San Francisco an 8-5 lead. He and Hundley both homered off left-hander Jake McGee.
Colorado lost for the third time in 11 road games this month.
San Francisco rallied from a 4-1 deficit.
The Rockies trailed 1-0 going into the fourth when Trevor Story's RBI double highlighted a two-run inning.
Colorado sent eight batters to the plate in a two-run fifth, chasing starter Ty Blach.
The Giants took an early lead when Buster Posey tripled and scored on Longoria's sacrifice fly in the first. Posey's triple was the ninth of his career and first since Sept. 10, 2016.
Arenado was 3 for 5 and finished 8 for 18 in the series. He has reached base safely in 28 of his last 29 games and 39 of 42 overall.
With two Giants second basemen on the disabled list, veteran third baseman Pablo Sandoval took grounders at second Sunday morning. He played two-thirds of an inning at second for Boston in April 2017. "He surprises me how quick his feet are and he's pretty good around the bag," manager Bruce Bochy said.
Giants: OF Andrew McCutchen was out of the lineup with a bruised knee and is day to day. ... 2B-OF Alen Hanson (left hamstring strain) won't travel with the team to Houston. Hanson is eligible to come off the disabled list Wednesday but is a few days behind his projected return, Bochy said.
Rockies: RHP German Marquez (2-2, 5.15 ERA) will pitch Monday's series opener in Los Angeles against the Dodgers. The 23-year-old is 2-2 with a 2.25 ERA in five road starts.
Giants: LHP Andrew Suarez (1-3, 4.88) starts Tuesday's series opener in Houston against the defending World Series champions. The 25-year-old rookie has struck out 28 and walked five in 27 2/3 innings.