noun (n)

• the cardinal number that is the sum of one and one and one(noun.quantity)

  Synonym:
  3, deuce-ace, iii, leash, tercet, ternary, ternion, terzetto, three, threesome, tierce, trey, triad, trine, trio, triplet, troika
  

  source: wordnet30
• the union of the Father and Son and Holy Ghost in one Godhead(noun.person)

  Synonym:
  blessed trinity, holy trinity, sacred trinity
  

  source: wordnet30
• three people considered as a unit(noun.group)

  Synonym:
  threesome, triad, trio
  

  source: wordnet30
• The union of three persons (the Father, the Son, and the Holy Ghost) in one Godhead, so that all the three are one God as to substance, but three persons as to individuality.(noun)

  source: webster1913