Klipsch Custom 3s are not a custom IEM. A real custom IEM will have the company send you a ear mold kit to get a molding of the inside of your ear or tell you to go to a audiologist (place where people go to get hearing aids) to get a ear canal mold so you can send that to them. Then they put the IEM inside a plastic (or similar) casing and ship that to you. Basically IEMs that are custom made for your ear canal and no one elses.
The Klipsch's Custom 3s is just a name.
Edit: The Klipsch Custom 3s use gel flange tips and gel biflange tips.