Sefer Chofetz Chaim Chapter 2 Halacha 7 Footnotes 15 & 16

Today we learned about keeping secrets. Even if lashon hara was said in a group and could inevitably spread but if they were told to keep it a secret then no one is ever permitted to reveal it. Even if one or two others broke the promise of secrecy the third cannot reveal it because without him it will not go public. The whole allowance of apei tlasa is because when there are three people who heard the news three people are enough to create a chain reaction of everyone hearing about it. If it is one or two people then a lot of people might wind up hearing about it but not everyone. Even so there are others reason of why not to repeat it. The Yad Ketana said it can only be repeated because the person telling it obviously didn’t mind it being repated if said in a group but in this case he does mind because he told everyone to keep it a secret. There is also a verse in Mishlei (11:13) which says “One who walk as a talebearer reveals secrets.” There seems to be an extra special prohibition of revealing secrets so even if it was told in a group of four and three of them broke the secret so everyone knows about it, the fourth one is still forbidden to tell it over to anyone.