Mmm. So if the youngest of the three is The Youngest, and the oldest of the three is The Eldest, how is the middle one described? Tsitsipas The Middle(st)?
Petros Tsitsipas, the middle of three brothers. Why does there have to be a specific description? In the case of three siblings, he or she is simply the one born after the eldest & before the youngest. CD could simply have referred to Petros Tsitsipas as Petros Tsitsipas or Tstitsipas (P) instead of trying to embellish.