How much time would you devot to Nice and area

Maybe I should try 4 days in the Nice area.

I am just working out my France trip.
It is mostly art centred as my 7 day bike tour is the areas of Provence that Van Gough worked.

I have to account for travel time.

To get from Nice to Paris does the train first stop at Marseille?

Is Marseille worth a stop?

With only 3 days you have to prioritize and consider your choices, and given the choices you'll probably be happiest doing all the things there are to do on the French and Monaco side, where there's more than enough to see/do for your 3 days.

I'm with Kent on this. The part of Italy you can reasonably get to, in a day from Nice, is not spectacular enough to warrant devoting 1/3 of your very limited time. I wouldn't spend a minute in Monaco either, but to each his own. However, since you indicated an interest in modern art, I recommend a visit to St.Paul de Vence. It's a medieval town (though now heavily impacted by tourism) which is home to Fondation Maeght. FM houses a fabulous collection of modern art. The sculpture garden is especially noteworthy, with works by an impressive array of modern masters.

The city of Nice has a very nice Museum of Modern Art which you wouldn't want to miss.

A beautiful, scenic walking town if you have time is Villefranche sur la mer on train route. But 3 days is not much for Fr Rivera highlights. you can train it easily and cheaply Nice, Monaco, Antibes.