It is not a secret after three people know it.
Note: An English variant is “The secrets of two no further will go, the secret of three a hundred will know.” Benjamin Franklin may have been paraphrasing this when he wrote, “Three may keep a secret if two of them are dead.” Shakespeare wasn’t sure even two could keep a secret:
Is your man secret? Did you ne’er hear say
Two may keep counsel, putting one away?
Romeo & Juliet
Gossip is such an integral part of Ireland’s oral tradition that this proverb is widely known in one of several variations, e.g.,
Ní scéal rúin é más fios do thiúr é.
(It is not a secret if three people know it.)
Chan scéal rúin a chluinneas triúr.
(A story that three people hear is not a secret.)
‘Rúin’ is the genitive singular case of ‘rún,’ meaning secret, mystery, purpose, intention, love, or affection. Obviously, it is an ambiguous noun. On the other hand, ‘scéal rúin’ is an idiomatic expression that unambiguously means secret. A secret story (the literal meaning of ‘scéal rúin’) is the stuff of gossip, not the usual arcane grist of espionage.