France, Germany and Croatia sounds quite enough for three weeks. I wouldn't try to squeeze in more countries, tbh. You could get out of Paris for a few days and experience rural France, which is beautiful. Or head to another French city, like Dijon.
If you insist I guess I could recommend my own country, the Netherlands, but then I would recommend other cities than Amsterdam. Good ones: Haarlem, Rotterdam, Leiden, Maastricht.