Hello! Sounds like an exciting trip to Sri Lanka you've got lined up. Here are some detailed travel tips for Jaffna, Mannar, and Trincomalee.

Best Time to Visit:
The best time to visit these destinations in Sri Lanka is from February to April, when it's relatively dry and the temperatures are moderate. December and January can also be good, but you might encounter some rain.

Route and Accessibility:
Jaffna is accessible by road, rail, and air. You could take a flight to Jaffna International Airport, or ride the train - the 'Yaal Devi' from Colombo. For Mannar, it's best reached by road. The drive from Colombo to Mannar takes about 5 hours. Trincomalee can be reached by road or rail as well, with regular bus and train services from major cities like Colombo.

Accommodation and Booking:
There's a wide range of accommodation options to suit every budget. For an authentic experience, consider staying in guest houses or homestays. It would be a good idea to book your accommodation in advance, especially during peak tourist season.

Clothing and Essentials:
Pack lightweight, breathable clothing as it can get quite hot. Don't forget your sunscreen, hat, and sunglasses. Carry a mosquito repellent as well, since the mosquito activity can increase during the monsoon season.

Health and Safety:
Consider taking travel insurance before the trip. Stick to bottled water to avoid any stomach issues. It's wise to stay updated on vaccines suggested for travel to Sri Lanka, such as Hepatitis A, Hepatitis B, Typhoid, Yellow Fever, etc.

Local Etiquette and Respect:
It's important to respect local customs and traditions. Dress modestly when visiting religious sites. Always ask for permission before photographing people. Lastly, avoid public displays of affection as they are considered inappropriate.