A key witness at the murder trial of a Hells Angels motorcycle club member charged with killing three men testified Wednesday that he helped dispose of the victims’ dismembered bodies. David Casey, 65, of Canaan, N.Y., testified in Hampden Superior Court in Springfield that he helped Adam Lee Hall bury the remains in a 4-foot-deep trench on private property in Becket in Berkshire County. Hall, 36, of Peru, Mass., and two other men are charged with fatally shooting David Glasser, Edward Frampton, and Robert Chadwell in Pittsfield in August 2011. Prosecutors said Hall wanted Glasser dead so he could not testify against him in an unrelated assault case. Casey is charged as an accessory.