MB 15: There are those who say that according to Rav Amram. They believe that the sewing is considered attaching, and only the tzitzis which were on the piece at the time it was detached and sewn back on are invalid because of "ta'asey v'lo min ha'asuyi" ("pre-fab tzitzis"). See the Taz who disagrees with the Shulchan Aruch, and writes that everyone admits that if after the sewing if one ties on tzitzis, it is kosher. The Bach writes this as well, and so does the Gra, the Artzos HaChaim, and others. Nevertheless, in order to fulfill the first opinion, one should sew under the corner of the garment a piece that is 3 fingers by 3 fingers.
MB 16: Fulfill all opinions. Therefore, we have a custom to sew on each corner of the garment a piece of material which measures 3 by 3 finger breadths, because in some garments, it is found that even in new garments there is sewing within the 3 fingers, and Rav Amram, according to the first opinion, would invalidate them (as we saw above in MB 12).
MB 17: Where possible. And when it is not possible (to fulfill all opinions), then we follow the opinion of Rav Amram, and also say that sewing is considered attaching.