The violence came as Xi was wrapping up what state media characterised as an "inspection tour" of the volatile region in China's far west, during which he had called for a "strike-first" strategy to fight terrorism.
"The battle to combat violence and terrorism will not allow even a moment of slackness, and decisive actions must be taken to resolutely suppress the terrorists' rampant momentum," Xi said in comments published early Thursday by the official Xinhua news agency.
Xinhua said two attackers slashed people with knives and set off explosives at the southern railway station in Xinjiang's capital Urumqi on Wednesday evening.
The agency called it a "violent terrorist attack" and said the suspects had "long been involved in religious extremism".
It said two of the dead had detonated bombs they were carrying, adding that the other fatality was an "innocent citizen".
Xinhua identified one of the alleged assailants as Sedirdin Sawut, a 39-year-old male from southern Xinjiang.
In the past, China has also pointed the finger at what it calls religious extremists with support from outside groups, but is careful not to blame the region's ethnic Uighurs in general.
The mayhem came just two months after machete-wielding attackers rampaged through a railway station in the southern city of Kunming, killing 29 people and wounding 143 in what many in China dubbed the country's "9/11".
Mass arrests claim
Dilxat Raxit, a spokesman for exile group the World Uyghur Congress, citing local sources, claimed more than 100 Uighurs were arrested in the aftermath of Wednesday's attack.
"Uighurs struggling between despair and survival expect Xi Jinping to come to East Turkestan to give constructive suggestions on improving the turbulent situation," he said in an e-mail, using the term for the region favoured by exile groups.
"However, the fact is Beijing continues encouraging armed suppression of Uighurs," he wrote.
He suggested the situation remained volatile, as "any Chinese provocation would directly incite further turbulence".
Xinjiang is a vast and nominally autonomous region where Uighurs are the largest ethnic group, though decades of migration to the area by China's dominant Han majority has fostered tensions.
Washington moved Thursday to condemn the attack, which it said "appears to be an act of terrorism that targeted random members of the public".
"The United States condemns the horrific and despicable acts of violence against innocent civilians at the train station in Urumqi in Xinjiang Uighur Autonomous Region of China," Deputy State Department spokeswoman Marie Harf told reporters.
"We offer our condolences and sympathies to the victims, their families and all those affected by this tragedy," she said.
'Fists and daggers'
Xinjiang is periodically hit by deadly clashes that authorities blame on terrorists but which rights groups and analysts say are driven by cultural and religious repression as well as economic disparities.
Explosives went off at 7:10 pm Wednesday, "centred around luggage left on the ground between the station exit and a public bus stop", Xinhua said, citing witnesses, without elaborating on the nature of the explosions.
It quoted one witness, a bread vendor, as having heard two blasts.
The station was closed after the incident and rail services suspended before being reopened at around 9:00 pm under the presence of armed police.
In another high-profile incident in October last year, three members of the same Xinjiang family crashed their car into tourists in Beijing's Tiananmen Square, killing two, before setting it on fire and dying in what authorities called a terrorist attack.
The timing of the latest incident came at a particularly sensitive and potentially embarrassing time for Xi, whose first visit to the region since taking power has received prominent coverage in state media.
It was unclear if Xi was still in Xinjiang when the attack occurred. Xinhua said only that his visit to the region ended Wednesday.
State television footage on Thursday morning showed Xi, clad in a black work jacket favoured by China's top leaders, meeting with law enforcement officials, ordinary residents and school children during his visit.
"Grassroots police stations are our fists and daggers so we must manage the basic level work well, care for our policemen, and you should protect yourselves well," Xi said while touring a police station.
"I hope you can make excellent achievements in the future work of serving the people and maintaining social stability."
Xi was also shown visiting a primary school, where he was greeted and given a traditional Uighur cap, or doppa, which was placed on his head.
Online, users of Sina Weibo expressed mixed views about the latest attack.
"We should love kind Xinjiang people and firmly strike evil terrorism!" one person said in a posting. "(We) have to let the terrorists come to a bad end."
Another, however, took a critical view of authorities. "If the CCP (Chinese Communist Party) doesn't improve people's well-being and win support from the majority, then counter-terrorism is a mission impossible," the post said.