The east is getting much warmer now and wildlife in Yala National Park is excellent, with leopards enjoying the sunshine. Other game is around too, including buffalo, crocodiles and elephants, as well as lots of birds. Udawalawe is also great at this time, home to about 700 elephants, as well as 30 or so reptile species. In both Yala and Uda Walawe, camping is an excellent option at this time.
To finish with a bit of beach relaxation is always a good choice, and the beaches of the south east coast are continuing to warm, with the average temperatures now up to about 35 °C. Bentota, Galle, Tangalle, Weligama and Mirissa are fantastic options, Weligama especially if you want to see blue whales.
All things considered, March is a fantastic choice if you want to explore Sri Lanka throughout. Combine your wildlife safari with some beach and enjoy the wonderful culture. You should definitely experience the best of the sunshine it has to offer at this time.