Where are you staying, not looking for the exact place, just the area.
If in Nice centre, and you have three days, first of all get your walking shoes on and do a bit of the Promenade des Anglais. Then get the little choo choo train from the city end of the Promenade (the hill is a killer!) up to the Chateau (Castle Hill), it is fab up there and the views are amazing. Walk back down.
You have to see the old town and the Cours Saleya (flower market) very atmospheric and lots of lovely restaurants. The tram will take you there. The main shopping area is on Ave. Jean Medecin and surrounding streets, again the tram gets you there. Very busy place...
Yes take the train to Monaco although I think it is very over rated and EXPENSIVE. But do go to say you've been there, we all did! I prefer the trip to Ventigmilia just on the Italian border, the Friday market there is good if you are into that sort of thing.
I would recommend at least a few hours in Cannes, take the train or bus. Yes it is high end, but there are ordinary people living there too! It is very nice. The area around Rue Meynadier is full of little shops and cafes and full of locals, that always impresses me!
Have a lovely time and pack it in!