Hello, I was mekabel a chumra in my mind, and I didn’t do it three times- is it a neder? Thank you.
The kabala of the neder doesn’t mean anything if you didn’t verbalize it, (according to most poskim) unless it was a neder to give tzedakah. However if you started keeping your kabalah and you had in mind to continue doing it, then even if you only did it once it is considered a neder.
Y:D 214-1, Sharei Teshuva 460 (towards the end of 10), Aruch Hashulchan Y:D 258-39, Igros Moshe Y:D 1-127 (8), Kol NIdrei 59 1,2,3.