Sure, I'm here to help you with your trip to Trier, Mainz, Xanten. Let's go through some important travel tips.

Best Time to Visit:
The best time to visit these cities: Trier, Mainz, and Xanten in Germany is from May to September when the weather is warm and pleasant for outdoor activities.

Route and Accessibility:
To reach these cities, the best way is to fly into Frankfurt International Airport, which has good connectivity to most parts of the world. From there, you can take the German Rail (Deutsche Bahn) to Trier, Mainz, and Xanten. These cities also have good connectivity through the Autobahn (highway system) if you prefer to drive.

Accommodation and Booking:
You will find a range of accommodation options in these cities, from luxury hotels to budget hostels. It's advisable to book your accommodation in advance, especially during the peak tourist season. You can use online platforms like Booking.com or Airbnb for reservations.

Clothing and Essentials:
Typically, summer clothing would be suitable for your trip considering the best time to visit. However, do pack a light jacket or sweater as it might get cooler in the evenings. Don't forget to pack comfortable shoes for exploring the cities on foot. Also, having an umbrella or raincoat might come in handy as these cities occasionally experience rain in the summer.

Health and Safety:
Germany is generally safe for tourists. However, it's always good to have travel insurance. Keep emergency numbers handy. For any health problems, pharmacies, known as 'Apotheke,' are widely present in these cities.

Local Etiquette and Respect:
Germans typically respect punctuality, privacy, and they follow the rules strictly. It's essential to greet people with a simple 'Hallo' before starting a conversation. Also, it's respectful to return any toasts made during meals or social gatherings.