The only thing left for Sri Lanka in this series is to avoid whitewash and this is possible only if they won third T20I. Sri Lanka played well in first T20I but New Zealanders dominated today's game. Today New Zealand gave target of 186 and Sri Lankans were 45 runs short of this target. Now pressure is on Sri lanka in the final game and they have the capability to win the last T20I to avoid whitewash. To win the last game, Sri Lanka need to control New Zealand batters and stop them from giving big target in case they are batting first.
Sri Lanka main goal now is to stop New Zealand from winning all games in series. They can only do this if they win third T20I game. In first match Sri Lanka played well but New Zealand played better in second game. Sri Lanka tried to catch up to New Zealand score of 186 but they were 45 runs short. Now Sri Lanka is under pressure to do well in last match but they are good enough to win and not lose all games. To win they need to stop New Zealand batsmen from scoring too many runs especially if New Zealand bats first.
Yes, one of the goals of Sri Lanka at present is that they do not get whitewashed in the T20 series, if they can win the third match against New Zealand, then they will win at least one match in the series. I think Sri Lanka will play well enough to make New Zealand lose their next match and they will try their best to beat New Zealand. If Sri Lanka can perform better in their third match compared to the last two matches, then it will definitely guide them to win. But of course Sri Lanka will have to put pressure on the New Zealand batsmen so that they do not score more runs, if they score more runs, then Sri Lanka will have less chance of winning. So if Sri Lanka can get the top order batsmen of New Zealand out early, then Sri Lanka will have more chance of winning.