and if you already claimed them, don't use them yet.
These are the reward for the last day of attendance.
They are 3 cubes, and they are together in a single stack (which is not something Meister's Cubes normally do).
If you use one, they all disappear.
Attendance rewards can be claimed until January 5th, and the cubes last 7 days.
Hopefully by then, Nexon will be back from their holiday vacation and fix them.