>>224
Strictly speaking, 年上 and 年下 implies the gap is greater than one year or they are/were in different years at grade school (Skipping/failing a grade is quite rare in Japan). So, in written language, I would say:
AさんはBさんより３ヶ月若いです
or use a different wording like:
Aさんの誕生日はBさんより３ヶ月早いです or 遅いです
when A is older or younger, respectively.
That said, 年上 and 年下 are loosely used especially in spoken language. So it's not very unusual to say:
AさんはBさんより３ヶ月だけ年下です
AさんはBさんより３ヶ月年上です.
だけ ("merely" in English) is placed to emphasize that the gap is less than one year. I omitted the word in the second sentence because using だけ can be impolite when you're referring to an elder person depending on the context. Also, 年上/年下 may sound funny/humorous when the gap is measured by weeks/days/hours/seconds. So a Japanese twin may make a joke like "私は弟より１時間年上です."