A morale-boosting win in the two-week event will kickstart preparations for the Champions Trophy to be held in India in October-November and the World Cup in the Caribbean in March-April next year.
"It's an important series to see where we stand," said Sri Lankan coach Tom Moody ahead of his team's opening game against South Africa under the Premadasa stadium lights on Monday.
"Any of the three teams can win. I am not discounting any side. The key will be to play consistently well over the next two weeks."
Mahela Jayawardene's in-form home team are on a roll after a successful tour of England earlier in the summer where they fought back to draw the Test series 1-1 and blanked the hosts 5-0 in the one-dayers.
Last week Sri Lanka drubbed South Africa 2-0 in a home Test series, during which Jayawardene hammered the fourth highest Test score of 374 and shared a world record stand of 624 with Kumar Sagakkara (287). Spin wizard Muttiah Muralitharan claimed 22 wickets in the series.
Sri Lanka appear almost invincible in their own backyard, having won 15 of their last 20 home one-day internationals with four defeats and one game being rained off.
Jayawardene, however, refused to dwell on the past and warned team-mates against being complacent for the tri-series.
"We have done a lot of good things over the last three months and the challenge is to maintain those standards," the 29-year-old said.
"We can't afford to be complacent. With all the technology available, teams will look at what we did and have their own plans to contain us."
India, who thrashed Sri Lanka 6-1 at home last year, hope to leave behind the disppointment of a surprise 1-4 loss to Brian Lara's unheralded West Indians in the Caribbean in May.
"Sri Lanka is a very improved side after England," said Indian captain Rahul Dravid. "The victory over them at home last year does not give us any advantage.
"It's not been easy to win in Sri Lanka. Hopefully we will change it this time, and we can if we play quality cricket."
India are boosted by the return of Sachin Tendulkar, one-day cricket's leading batsman with 14,146 runs and 39 centuries, after a five-month absence due to surgery on his right shoulder.
South Africa, the number two team in the official one-day rankings behind world champions Australia, have been battered by a string of injuries to key players.
Already without captain Graeme Smith, star all-rounder Jacques Kallis and hard-hitting batsman Justin Kemp, the Proteas are concerned over pace spearhead Makhaya Ntini's hamstring strain and veteran Shaun Pollock's lower back pain.
"Makhaya is 50-50 for the first game but Shaun should be fit to play," said wicket-keeper Mark Boucher, who will captain South Africa in the one-dayers after Ashwell Prince led in the Tests.
South Africa's last one-dayer in April saw them surpass world champions Australia's 434-4 off the penultimate ball in a sensational one-wicket win at the Wanderers in Johannesburg.
Herschelle Gibbs, who smashed 175 off 111 balls in that match, will spearhead the South African batting in the absence of Smith and Kallis.
Each team plays the other twice in the league with the top two advancing to the final on August 29.