The Operation 4 update canceled Scrap & replaced it with another currency (Gears Coins) to purchase items from the store.
To get this achievement you need to acquire (using Gears Coins or Iron) any 3 different characters skins that are not related to a launch character.
Any skin of the following characters WILL NOT COUNT since these are the launch characters:
1- Swarm Drone
2- Swarm Grenadier
3- Swarm Hunter
4- Swarm Sniper
5- Swarm Imago
7- The Speaker
* Some people may get this achievement after buying only 1 or 2 skins, or not buying any skin at all.