Where should I plan my first 3 days after a brutal long haul to avoid wasting them to jetlag

Plan a slow landing zone, not a dash across the country. Spend the first 2 nights in Negombo, just 10 to 15 minutes from the airport. Mornings on the beach, afternoon naps, sunset at the lagoon, fish market at dawn on day 2 (a great gentle "first culture hit"). On day 3, do a short, easy transfer (no more than 3 hours) to one place, ideally the Cultural Triangle (Sigiriya area, ~3.5h with a slow driver), where you can either chill at an eco-lodge or do a half-day at Dambulla cave temple. Do NOT plan Adams Peak, Yala safari or a 7-hour Ella train on day 1 or 2; you will hate yourself. Drink water relentlessly, expose yourself to morning sunlight, and avoid alcohol the first 48 hours.