Sure! Travelling to Chiang Mai, Chiang Rai, and Phetchabun in Thailand is an exciting expedition.

Best Time to Visit:
The best time to visit Chiang Mai and Chiang Rai is from November to February when the weather is relatively cool and comfortable. For Phetchabun, the best time to visit is from November to January when the overall temperature is cooler due to the winter season.

Route and Accessibility:
These three cities are well-connected by road and air. There are daily flights from Bangkok to both Chiang Mai and Chiang Rai. Buses and trains are also readily available from other parts of Thailand.

Altitude and Acclimatization:
These areas are not located at very high altitudes so there is no need for acclimatization. Most visitors should not feel any negative effects.

Accommodation and Booking:
Thailand's tourism industry is well-developed, so online hotel booking is accessible and provides a wide range of choices. It is advisable to make bookings in advance, especially during peak seasons.

Clothing and Essentials:
Light clothing is sufficient for most of the year, but during the cooler months (November-Febraury), you might need a light sweater. Good walking shoes are recommended.

Health and Safety:
Thai food can be spicy for some, it's best to state your preference while ordering. Stay hydrated and use sunscreen. It's recommended to have travel insurance with medical coverage.

Local Etiquette and Respect:
Show respect to local culture and religious sites. Dress modestly while visiting temples (No short skirts, shorts or sleeveless tops). Removing shoes before entering someone’s home or a temple is mandatory in Thai culture.