Function for a specific numeric sequence without any parameters

I got an university task which i am totally stuck on. Normally I wouldn’t post about it here, but none of my colleagues has any idea how to solve it.
The Task:

Design an algorithm "NEXT", which delivers the subsequent value of the numeric sequence {0,3,5,6,9,10,12,15,18,20,21,…} as function value whenever it is called. The algorithm must not use parameters or global variables or vectors.

I have to implement it in C++ (which shouldn’t be too much of an issue for me). I just have no idea how. I can’t find any form of rule for the number sequence. And i am kinda stuck on the part where i have to return the next value of the sequence without giving any parameters to the function.

It would be awesome if someone could help me out here 🙂

(FYI, i had to translate the task and i hope I didn’t make any critical Errors… Here it is again in the original languge german:
Entwerfen Sie einen Algorithmus N E X T, der bei jedem Aufruf den
jeweils nachsten Wert der Folge {0,3,5,6,9,10,12,15,18,20,21,…} als Funktionswert liefert.
Der Algorithmus darf weder Parameter haben noch globale Variablen oder Vektoren verwenden.)

>Solution :

The sequence consists in all multiples of 3 or 5, in increasing order.

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