Ok great. Basically stick it somewhere in the sump with high flow (I rinse mine first), but don't replace either of the other two with it.
Purigen (or any organic resin filtration like what's part of chemipure blue) is seperate to both carbon and biofiltration. You can think of purigen as carbon, but targeted towards different types of chemicals. Carbon will work on a lot of the same things as purigen, but not as well. Purigen is particularly good at filtering dyes and longer chain polymer (IIRC).
Unless there's extenuating circumstances, you dont really need to replace your biofilyer media. It's just surface area for bacteria and other things to grow on.