Aa the excellent article points out only two triple Niña have emerged since 1950, both after a very strong El Niño. However, from Eric Webbs ENS ONI stretching back to 1850 there are indications that a triple La Niña developed in 1908-1911. ..I.e started by fall 1908 and lasted to spring 1911. That La Niña evolved after a moderate double Niño in 1904-1906 followed by neutral conditions from summer 1906 until summer 1908.

https://www.webberweather.com/ensemble-oceanic-nino-index.html