What is the source for the idea that doing some good thing 3 times constitutes a vow? I looked through Hilkhot Sevuot and Nedarim, and in the Entziklopedia Talmudit under Hazaka, and I could find no such idea.
olamot.net/shiur/%D7%91%D7%9C%D7%99-%D7%A0%D7%93%D7%A8– Gershon GoldSep 21, 2015 at 19:15
I believe it is a chidush of the Ran on Nedarim.– LoewianSep 21, 2015 at 22:33
I'm not sure about an original source, but it's mentioned in Hatarat Nedarim done on Erev Rosh Hashanah:
או אֵיזו הַנְהָגָה טובָה או אֵיזֶה דָבָר טוב שֶׁנָּהַגְתִּי שָׁלשׁ פְּעָמִים וְלא הִתְנֵיתִי שֶׁיְּהֵא בְּלִי נֶדֶר (text source)
... Or any good act or good thing that i have done three times and did not stipulate that it should not be a neder ... (my own translation)
2This is not a great answer. It doesn't mention the origin of the notion. The siddur version of Hatarat Nedarim, was formulated far later than the concept of the 3 time practice being a form of vow. I.e., the text formulation just states that it requires absolution, but obviously, it is extracting that notion from somewhere else, possibly from the Gemarah. This would be more appropriate as a comment.– DanFSep 21, 2015 at 21:09
The Imrei Yosher 2:136 uses the Hataras Nedarim Erev Rosh Hashana as a primary source, so the answer doesn't deserve the negative attitude, IMO.– YishaiSep 21, 2015 at 22:46
If a person decides to intentionally do something which is a "chumrah" (voluntary strict practice, not required by law) and also intends that his practice be his new way of acting forever, (AND he is well aware that it is not obligatory,) then it has the halachic force of a neder (vow) even if he only does it one time. (see Shulchan Aruch Yoreh Deah 214:1; based on Talmud tractate Nedarim 15a.) The novelty here is that the vow takes effect without a verbal declaration. (D'Rabbanan)
If a person had in mind that the good practice would be a one time affair (or for the next 4 times for example, but no further) then that is OK. (This means that a person can limit the vow by such parameters.) If he had in mind that the practice would specifically not be binding, then it is not a vow at all. If a person accidentally thought that what he was doing is obligatory, then we pasken like the authorities there (214:1) that there is no vow.
The Gemara and Shulchan Aruch in the source does not mention 3 times.
Rav Shlomo Ganzfried (1804 - 1886) writes in Kitzur Shulchan Aruch 67:7, that in addition, if the person did not intend to have his new practice be from now on and forever, but had nothing particular in mind, then we apply the rule that if he did it 3 times (without having in mind that it is not a vow,) then it becomes a vow.
Other Rabbis and prayerbook authorities (commenting on hataras nedarim in the machzor) apply this rule like the Kitzur SA does. It is based on the principle of a "Chazakah" being created by anything done 3 times. (See Talmud Baba Basra - beginning of 3d chapter, for the general halachic concept of 3 times being a chazakah.)
Finally, it should be noted that there are therefore halachic leniencies regarding such a vow, such as special instances when the vow might not be in force, and how to retract such a vow in front of a Rav. As always, one should consult their LOR.
Thank you! I would just clarify that the ShA 214 specifies that this is about prohibiting oneself some thing (a neder), not undertaking to do something (which could be a sh'vua), and I meant to ask about the latter. Sep 22, 2015 at 15:40
I will provide sources later. But it's really a recent innovation. It's not Mishnaic, Talmudic, nor Geonic. Rabbi Dovid Fink gave an excellent course that mentioned this exact topic and if I can find the link later I'll post it.