The experts are, Martti Ahtisaari, former President of Finland and Nobel Peace Prize Laureate, Silvia Cartwright, a former Governor-General and High Court judge of New Zealand, and Jahangir, former President of Pakistan's Supreme Court Bar Association and of the Human Rights Commission of Pakistan, according to a UN statement.
The investigation will look into alleged serious violations and abuses of human rights and related crimes by both parties in Sri Lanka during the last years of the armed conflict that ended in 2009.
"Each of them brings not only great experience and expertise, but the highest standards of integrity, independence, impartiality and objectivity to this task."
The investigation team with whom they will work will consist investigators, forensics experts, a gender specialist, a legal analyst and other specialised staff.
More From This Section
It will be operational for a period of 10 months (up to mid-April 2015).
Responding to the Sri Lankan government's stance of non-cooperation with the investigation, Pillay said, "Once again, I encourage the government and people of Sri Lanka to cooperate fully with this investigation which can help shed light on the truth, and advance accountability and reconciliation in Sri Lanka".
Sri Lanka regards the investigation as intrusive and an attack on the island's sovereignty.
Up to 40,000 Tamil civilians were killed in the last months of the brutal civil war in 2009, rights groups and experts have alleged.
In a US-backed resolution at the UNHRC session in Geneva in March, the UN endorsed Pillay's recommendation for an external probe into the alleged war crimes in Sri Lanka.
In 2012 and 2013, India had voted against Sri Lanka but this year it abstained from voting on the resolution.