3 night in Nice

Buy this book. It answers all your questions. By the way, the country is spelled Monaco.

https://www.amazon.com/Rick-Steves-France-2018/dp/1631216686/ref=sr_1_1?ie=UTF8&qid=1521297427&sr=8-1&keywords=rick+steves+france+2018+guide+book

There is also a dedicated RS guide to the French Riviera (and Provence).

Monaco can be reached via bus (100) or train from Nice. It's very easy to navigate on your own, and Monaco is small. Please remember to dress appropriately for the Casino, if you want to go in. Often, people combine the trip with a journey to Eze, which has a spectacular view.

Also consider a trip to VilleFranche Sur Mer and Cap Ferra (Villa Ephrussi). The harbor at Antibes (via train, do not take that slow bus) is also known for big yachts, and there are usually some in Nice's Old Port. Some people hit Antibes on the way back from Cannes. In Nice, be sure to allocate some of your time to the Old Town, Castle Hill and Cours Saleya Market and a walk along the Promenade (lunch on the beach at the Beau Rivage or the new Galion Plage).

Another day trip from Nice is St. Paul de Vence (bus 400) and the Fondation Maeght. I do recommend that you go to one of the smaller villages (Eze, Villefranche, St. Paul de Vence), to get a totally different feel than the two big cities.

If you'll only be in southern France, it would probably be better to get Rick's book that focuses on that region. It is more comprehensive.

https://store.ricksteves.com/shop/p/provence-guidebook

Thanks for the suggestions. Will definately go to Eze and I have also ordered Rick's book. Gee 3 different spellings of Monaco in one paragraph. Must have been multi tasking at the time. Sorry about that.