Mishnah 3. A brief introduction to understanding the mishnah: as was explained in the previous mishnah, the Gemara sets these cases in a woman who took a vow not to derive benefit, and her husband upheld her vow - instead of annulling it and nullifying it, he let it take effect and declared it valid. From this we learn that he hates her, and therefore she wishes to be released immediately; and we enable this for her by having him give her a get and pay her kesubah.
The case in the mishnah:
The mishnah uses the language of a man who imposes a vow on his wife "shelo tiskasheit" - not to adorn herself. The simple reading is that it was the husband who vowed against his wife, but in truth, as has been explained, she is the one who vowed and he upheld her vow: that she not adorn herself with any of the various adornments she is accustomed to, and not use perfumes and the like.
The law is that he must immediately give her a get and pay the kesubah: she will want to get out of this situation - assuming that she indeed wants to leave - and he becomes liable for the kesubah because he upheld the vow.
The view of Rabbi Yosi:
Rabbi Yosi holds that the matter depends on the woman's circumstances:
With poor women: who do not have adornments and perfumes in any significant quantity - he is required to divorce her immediately and give her a kesubah only if no time limit was set for the vow, but rather it would continue on indefinitely. But if a time was set for it, we wait until that time arrives. The Gemara explains that the time limit is twelve months, and in practice we wait twelve months.
With wealthy women: who have perfumes and adornments in abundance - we wait thirty days. The Gemara explains that a wealthy woman is able to enjoy her perfumes for thirty days.
And once that time has passed, he is again obligated to give her a get and pay her kesubah.
In summary: in this mishnah we learned that one who upholds his wife's vow not to adorn herself - we compel him to divorce her and pay the kesubah immediately; whereas according to Rabbi Yosi the matter depends on the duration of the vow and on the woman's circumstances: with poor women twelve months, and with wealthy women thirty days, and only afterward does the obligation of the get and the kesubah take effect.