triumvirate vs troika

triumvirate

noun
  • An official group of three people, especially a ruling council of three men and particularly (historical) two such councils in Roman history. 

troika

noun
  • A party or group of three, especially a ruling council of three people in Russian contexts. 

  • A Russian carriage drawn by a team of three horses abreast. 

How often have the words triumvirate and troika occurred in a corpus of books? (source: Google Ngram Viewer )