The victims -- two Colombians and a Frenchwoman -- perished when a device exploded in a ladies' restroom in the crowded Andino shopping center in Bogota on Saturday. At least nine people were also wounded, officials said.
President Juan Manuel Santos called the incident a terrorist attack.
Rebel groups condemned the blast and said it was an attempt to undermine their efforts with the government to end Colombia's half-century civil conflict.
"There was a strong boom and the floor shook," said shop worker Milena Carcenas.
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"There was smoke coming out of the bathroom. People were coming out of there covered in ash."
National police chief General Jorge Nieto told reporters "a device" was placed "behind one of the toilets in the women's bathroom."
Authorities have "three concrete hypotheses" on the perpetrators, Santos said Sunday after meeting with investigators, but declined to elaborate to avoid harming the probe.
There was a 100 million peso (about $33,000) reward for "anyone who can give us information to help capture those responsible," he said.
The last active rebel force, the National Liberation Army (ELN), meanwhile, has started talks with the government, though confrontations with state forces have been continuing.
"Those who want to rain on the peace parade will not succeed," said Santos, who won a Nobel Peace Prize last year for sealing an accord with FARC leaders.
"If this (bombing) is that kind of gesture, then rest assured that we will pursue those enemies of peace without rest and without quarter," he said, speaking at the site of the blast.
The peace deal was initially narrowly rejected by Colombians in a referendum, with critics saying it was too lenient on the FARC.
A redrafted agreement from Santos and the FARC has since been pushed through congress.
Bogota Mayor Enrique Penalosa called Saturday's incident "a cowardly terrorist attack."
He said the Frenchwoman who died, aged 23, had spent six months working in a school in a poor neighborhood.
The leftist ELN said on Twitter it "condemns this deplorable incident," noting that the attack was "against civilians."
The leader of the communist-inspired FARC, Rodrigo Londono -- known as Timochenko -- also denounced the explosion.
"This act can only come from those who want to close the roads of peace and reconciliation," he wrote on Twitter.
The blast was the second major attack this year in the Colombian capital.
In February, the ELN claimed responsibility for a bombing at a bullring in Bogota, which killed a police officer and wounded more than 20 people.
Colombia's civil conflict erupted in 1964 over land rights. It drew in leftist guerrillas, right-wing paramilitary groups and state forces.
But he told AFP: "There are some far-right paramilitary- style groups who have been responsible for killing civil leaders and for actions against the peace effort."
Efforts to disrupt the process, however, is unlikely, he said, citing public support for peace.