Hello! It's wonderful to hear you're planning a trip to France, covering the beautiful cities of Paris, Marseille, and Nice. Here are some expert travel tips for your adventure.

Best Time to Visit:
The best time to visit France, specifically locations like Paris, Marseille, and Nice, is either in the Spring (April to June) or Autumn (September to November), when the weather is moderate. These months also tend to be less crowded than the peak holiday season of summer.

Route and Accessibility:
If you're traveling from outside Europe, you'll likely fly into Paris' Charles de Gaulle Airport. From there, France boasts an efficient high-speed train system - you can catch the TGV (Train à Grande Vitesse) from Paris to both Marseille and Nice. You can also opt for domestic flights or rent a car for more freedom.

Accommodation and Booking:
These cities have a wide range of accommodation options to suit all budgets, from luxury hotels to budget hostels, and vacation rentals. It’s advised to book accommodations in advance, especially during peak seasons. Websites like Booking.com, Expedia, and Airbnb are commonly used.

Clothing and Essentials:
Depending on the season, you’ll want to pack accordingly - lightweight clothing for summers, warm clothing and a good coat for winters. Don't forget comfortable shoes for all the walking. Always pack an umbrella as rain showers could surprise you.

Health and Safety:
No specific health concerns relating to these destinations. However, it's always wise to have travel insurance in place. Keep an eye on your belongings in crowded areas to avoid pickpockets. It's advisable to keep emergency numbers handy.

Local Etiquette and Respect:
French people value their language so attempt to greet and thank in French. Always say 'Bonjour' when entering shops or cafes. It's also customary to say 'Bon Appetit' before meals. Tipping is not mandatory but appreciated for good service.