If you have 3 weeks I would visit the follwoing places:
must:
venice,paris,rome (sightseeing)
maybe:
amsterdam,barcelona,moscow, london
A must do is Switzerland, depending how much you like nature and hiking, decide how long you want.
If you like to chill at the beach go to southern france is very nice. YOu can stay in Nice and travel to Monaco,Cannes or just chill out.
I just went and hope to post a small tripreport soon. (Maldives is ij'h also coming)