3‐in‐1 suitcase set typically includes three sizes of hard‐shell luggage made from polypropylene (PP) — often marketed as unbreakable rubber because the material is strong, flexible, lightweight, and resistant to cracking during travel.
Some sets also mention double‐zip or anti‐theft zippers for extra security — meaning the zipper system is designed to resist being forced open easily and helps protect your belongings.