The Collaborative International Dictionary
Trigamist
Trigamist \Trig"a*mist\, n. [See Trigamy.] One who has been married three times; also, one who has three husbands or three wives at the same time.
Wiktionary
trigamist
n. 1 A person who has three wives; someone who commits trigamy. 2 A person who has a third marriage.