1. You Don’t have to sit for long hours at a single place. Why you want to do that ?
2. Keep your sitting duration between 40-60 mins or upto 90 minutes and after each sitting do basic stretching of back and other parts
3. In exam if you feel uncomfortable in sitting for three hours then don’t torture yourself daily by doing long sittings
4. In exam you should break the sitting period in two halves by taking a break after 90 mins for stretching your body for 2 mins ( go washroom or have some water)
5. If you wish to sit for long hours as a challenge to increase your ability you can do a 3-4 hours study session by keeping other sessions short ( you can do this once in a day or on alternate days).