When looking at Sri Lanka as a whole, March is often considered to be the optimum time to travel as the bulk of the island is still enjoying its dry season and the north-eastern monsoon is starting to wind down. So across Sri Lanka you should expect hot and dry conditions.
If you enjoy walking then this is a great time of year to head to the hills around Kandy (Knuckles Mountain Range) or Nuwara Eliya (Horton Plains and Adam’s Peak) then this month offers the best chance to enjoy both warm and dry conditions.
For your beach stay the south and south west coast still offer the best weather, but as the month draws to an end, if you’d like to get off the tourist trail the east coast beaches start to dry up.