Absolutely, I'd be glad to help you plan your visit to Portsmouth, Salem, and Burlington!

Best Time to Visit:
For Portsmouth and Salem, the best time to visit is late spring, summer and early fall, specifically May through October, when the weather is warm and numerous outdoor activities and events are on offer. Burlington's peak travel season is during the fall foliage, from mid-September to mid-October, but it's also pleasant in summer.

Route and Accessibility:
All three cities are well-connected by road and air. You can fly into Boston Logan International Airport or Manchester-Boston Regional Airport and drive to Portsmouth. From there, Salem is just an hour's drive. To reach Burlington, you can take a domestic flight from Boston or drive, although it's a longer journey of around 3.5 hours.

Accommodation and Booking:
From budget options to luxury hotels, there are plenty of accommodations available in all three cities. It's advisable to book in advance, especially during peak seasons. Online booking platforms like Airbnb, Booking.com, or direct hotel websites can be used for bookings.

Clothing and Essentials:
The weather in these cities can be quite unpredictable, so it's recommended to pack layers. A good rain jacket, comfortable walking shoes, and sunscreen are essential. Don't forget to pack a camera to capture the beautiful landscapes and charming cityscapes!

Health and Safety:
While these cities are generally safe, it's important to be aware of your surroundings and avoid isolated areas late at night. It is also recommended to have travel insurance for the duration of your visit for any unexpected emergencies.

Local Etiquette and Respect:
When visiting these cities, respect for local culture and customs is key. Americans appreciate punctuality and personal space. Tipping is customary in restaurants, bars, and for services like taxi rides or tours. In terms of environmental responsibility, ensure to leave places as you found them.