I came across this after seeing that the QM2 had set sail on it's maiden voyage. It seems the best known of the three wasn't the only one to end up on the sea bottom, just the best known.

] Titanic was the middle ship of the three new
] super-liners. Her older sister, Olympic, served as a
] reliable member of the White Star fleet until she was
] scrapped in 1935 after striking and sinking the famous
] Nantucket lightship off the eastern cost of the United
] States. Her younger sister, Britannic, met a fate similar
] to that of Titanic during World War I when she struck a
] German mine off the coast of Greece and sank in less than
] an hour. Britannic was originally to be named Gigantic,
] but further reference to size in the wake of the Titanic
] disaster was thought by White Star to go against public
] sentiment.