The teams are expected to formally announce the deal after the NFL trading period begins Friday.
Bly has been selected to the Pro Bowl twice in his eight-year career and is expected to take the place of Denver's Darrent Williams, who was killed on a shooting Jan. 1. He has 31 interceptions over his career, which has been spent with Detroit and St. Louis.
Bell piled up 1,025 yards on 233 rushes last season while splitting playing time with Mike Bell. He's going into his fourth NFL season.
Foster is a fifth-year player whose entire career has been with the Broncos.