A daylight bust in Downtown Cross netted 172 grams of heroin, 48 grams of crack, $931 in cash and a 9mm semi-automatic handgun.

Boston police said the Drug Unit “witnessed a drug exchange involving” three suspects and moved in to break it up.

Keith Curtis, 38, of Athol, Katherine Isaacs, 33, of Weymouth, and Michael Groden, 44, of Needham, were all arrested on charges related to firearms and narcotics, police said.

Curtis faces charges including unlawful possession of a firearm, unlawful possession of ammunition, trafficking class A and B substances, distribution of a class A substance, and conspiracy to violate drug laws.

Isaacs was arrested for trafficking class A and B substances, distribution of a class A substance, possession of a class B substance, and conspiracy to violate drug laws.

Groden was arrested for trafficking class A and possession of a class B substance. All three suspects are scheduled to appear in Boston Municipal Court for arraignment.