Delos Santos was a 17-year-old student who was gunned down by plainclothes police officers last week in the city of Caloocan in Metro Manila.
The accused officers have denied any wrongdoing, saying Delos Santos pulled a gun, forcing them to open fire.
An autopsy shows he was shot twice in the head and once in the chest.
A Senate Inquiry into Delos Santos' death was opened on Thursday after protests against what some consider the extrajudicial killing of an innocent child.
Delos Santos is one of at least 3,400 alleged dealers and users killed in a bloody crackdown on illegal drugs in the Philippines since Duterte took office in June last year.
The teenager was among 82 people killed by police on the night of August 16
in a new initiative, dubbed "one time, big time;" a heavy-handed push to eradicate drug figures in specific neighborhoods.
The three officers involved in the killing of Delos Santos are set be charged with "serious irregularities in the performance of duty,"
according to a report in the state-run Philippines News Agency (PNA).
Their commanding officer, Chief Insp. Amor Cerillo, was removed from duty, the report added, citing Allegar Triambulo, the head attorney for the Philippine National Police-Internal Affairs Services (PNP-IAS).
A heavy police presence guarded the Senate building Thursday, with officers stationed both inside and out as a small protesters called for justice for Delos Santos.
The murdered teen's family attended the hearings, with his mother weeping throughout the proceedings.
Police witnesses alleged that, per the testimony of a police informant, her son had been acting as a drug runner.
The officers involved they invoked their rights to self-incrimination, refusing to answer questions from senators during the hearing.
Swell of public unrest
Delo Santos' death could potentially turn the tide in Duterte's drugs war, which rights groups have decried as nothing more than brutal, extrajudicial massacre.
In the week since Delos Santos was killed, Filipinos have taken to the streets in greater numbers than ever before to demand justice for what they say is an unjustified killing.
Several protests have been held, including at the People Power monument, a memorial to the peaceful protests that ousted former dictator Ferdinand Marcos.
During that protest, on the anniversary of former president Ninoy Aquino's death, a small, determined band of demonstrators, young and old, came out in the rain to protest Delos Santos' death and with it, the bloody crackdown that the country has experienced since Duterte rode to office on a trail of angry, vengeful rhetoric.
On Wednesday, still more gathered outside the Philippine National Police headquarters in Manila to demand justice for Delos Santos, waving signs that read "stop killing the poor!" and "justice for all victims of EJKs," using the shorthand for "extrajudicial killings," a phenomenon that has become all too commonplace in the country's poor neighborhoods.
Jose Manuel "Chel" Diokno, one of the Philippines' foremost human rights lawyers and chairman of the Free Legal Assistance Group (FLAG) called the protests "significant" and added that, as more people speak out, "even the government is paying attention the reaction (to Delos Santos' death)."
He said that the "grip of fear is loosening" in the Philippines, and said the reason the public has until now been quiet about extrajudicial killings, is "not because they like it but because they're afraid that if they publicly come out they might be targets themselves."
"Public opinion is changing, people are more willing to speak out. As more abuses come to light, more will oppose it in public."
He said the change in the public mood is why a majority of senators agreed to hold the inquiry into Delos Santos' death.
"Politicians in the Philippines are like those anywhere, they are sensitive to public opinion. (Now) they sense a change in public opinion and that explains to me why the senators agreed to a hearing."
Although Duterte has a long history of tolerating -- even encouraging
-- his police officers to kill drug suspects, he said that the Caloocan police who slayed Delos Santos had gone too far.
"You are not allowed to kill a person who is kneeling down, begging for his life. That is murder," he said. "Let us be clear on this."
He said that, after seeing the footage, he called Dela Rosa, demanding that the killers be arrested.
"(The media was unaware), but right after it happened, I called Bato. 'Arrest the guys and place them in jail to wait for inquest.' (Push through with the case) if it's murder," he said, referring to the PNP boss by his nickname, according to
Vice-President Leni Robredo, a vocal critic of the administration -- Filipino vice presidents are voted for separately from the president -- visited Delos Santos' wake on Sunday, and was shown the place where he was killed,.
CNN Philippines reported that
she has offered assistance to the dead teens' family.
"Now, this (war on drugs) has a face, Kian. And the question on my mind is -- how many Kians will happen?" she asked.