Of the three sisters in the Olympic class, Titanic was short lived but probably the most famous ship in history, Olympic soldiered on after WW1 and was converted to oil burning to reduce operating costs. The Great Depression caught up with her and when no-one wanted to emigrate to America, she became uneconomic to operate (along with the ex-German liners of the same vintage) and was scrapped.
Britannic was completed just before the beginning of WW1, becoming HMHS Britannic 1915 and she was sunk in the Aegean, off the island of Kea, on 21st November 1916. The probable cause was a ine but fortunately only 30 lives were lost while over a 1000 passengers and crew were rescued. Nevertheless, Britannic was the largest ship sunk in the first world war.