I'm assuming you're using V1/2 clothing on V3. The V3 mesh is different enough that old clothes won't necessarily fit V3 properly. I usually parent the V1/2 clothes to V3's hips then after I pose V3, I apply the same pose to the clothes. I then then tweak the pose and play with morphs to cover up pokethrough. I can use some of Koshini's clothes on V3 with the same method. Cheers!