trigamous
Concise Oxford English Dictionary © 2008 Oxford University Press:
trigamous /ˈtrɪgəməs/
▶adjective having three wives or husbands at the same time.
– derivatives
trigamist noun,
trigamy noun.
trigamist noun,
trigamy noun.
– origin C19 (earlier (C17) as trigamist and trigamy): from Gk trigamos (from tri- ‘three’ + gamos ‘marriage’) + -ous.
'trigamous' also found in these Oxford entries:

