There are several boards featuring memory quilts made out of clothing items on Pinterest -- though if I recall correctly, most of them are baby/toddler themed.
I would think you'd need to wait and see what specific things you are given to work with before you can plan too much. Do you think they want the 3 pieces to work together as a set or can they be different? You might group the items by colors, or by textures. I think it would be useful to establish size of wallhaning before you start-- not everyone has expansive wall space, and it would be a shame to make something that has no place for display. You might also want to ask how they plan to hang it, and work with that end goal in mind as you design.