According to Harav Musafi Shelit"a (Q&A 74259) (and I'm sure earlier sources), if you do a "Hanhaga Tova - good deed" three times you need Hatarat Nedarim. I believe Harav Ovadia Shelit"a in his book Yechave Daas holds that this only applies for Hanhagot mentioned in the Shulhan Aruch. Can anyone provide the source in Yechave Daat where he says this?
In Yechave Da'at 2,70 R' Ovadia discusses women's obligation to keep their minhag of keeping Mitzvot Asseh Shehazman Graman. He generally holds that since they are not obligated - the fact that they kept something 3 times does not make it a neder. The is also a similar and longer response on Yabia Omer 2 Orach Chayim 30.