If you can't put the 3 quest items together, check if there is anything else in your inventory that can be combined. If there is and it already has the little plus sign on it, you need to seperate them first. There can only be one plus sign at a time in your inventory so if there already is another one, you cannot combine the quest items.

Top to bottom V, O, Catherine

[GER] Gelb, Blau, Grün
[ENG] Yellow, Blue, Green