tritagonist
Concise Oxford English Dictionary © 2008 Oxford University Press:
tritagonist /trʌɪˈtag(ə)nɪst, trɪ-/
▶noun the person who is third in importance, after the protagonist and deuteragonist, in an ancient Greek drama.
– origin C19: from Gk tritagōnistēs, from tritos ‘third’ + agōnistēs ‘actor’.
'tritagonist' also found in these Oxford entries:

