Three size 12 AWG two conductors with ground type NM cables are spliced in a ceiling junction box that has internal cable clamps and a flat cover. What is the minimum trade size metal octagonal box that may be used?

Three size 12 AWG two conductors with ground type NM cables are spliced in a ceiling junction box that has internal cable clamps and a flat cover. What is the minimum trade size metal octagonal box that may be used?